Nuclear well logging data acquistion system and method

ABSTRACT

A computer-based system and method for acquisition of nuclear well logging data, including downhole generation of energy and time spectral information. For acquiring energy spectrum data, a downhole multi-parameter analyzer is provided including a pulse height analyzer and memory. The analyzer converts each gamma ray pulse height into a digital code by peak detection, level and pulse pile-up discrimination, sample-and-hold, and analog-digital conversion. The digital code includes the address of a unique memory location or channel corresponding to the energy of the particular gamma ray. After a preselected time interval the memory channels collectively contain counts for each incremental energy band in the desired energy spectrum. The memory may then be interrogated by the downhole CPU for presentation of a gamma ray emission versus energy level plot. For acquiring temporal spectrum data, the multi-parameter analyzer also includes a memory address generator. The generator repeatedly produces a numerical sequence of memory address codes corresponding to a sequence of adjacent time windows relative to a reference start time, whereby the windows collectively comprise the time interval of the desired spectrum. Each time a gamma ray pulse is detected, the memory address generated at that time addresses a corresponding memory location and increments the count value resident therein. At conclusion of the time spectrum interval of interest the memory locations may be interrogated by the CPU and presented visually as a gamma ray emission count versus time plot.

BACKGROUND OF THE INVENTION

The present invention relates to hydrocarbon well logging systems andmethods; more particularly, it relates to a computer-based system andmethod for acquisition, presentation, processing, and recording ofnuclear hydrocarbon well logging data.

Well logging systems have been utilized in hydrocarbon exploration formany years. Such systems provide data for use by geologists andpetroleum engineers in making many determinations pertinent tohydrocarbon exploration. In particular, the systems provide data forsubsurface structural mapping, defining the lithology of subsurfaceformations, identifying hydrocarbon productive zones, and interpretingreservoir characteristics and contents. Many types of well loggingsystems exist which measure different formation parameters such asconductivity, travel time of acoustic waves within the formation, andthe like.

Still another class of systems seeks to measure incidence of nuclearparticles on the well logging tool from the formation for purposes wellknown in the art. These systems take various forms, including thosemeasuring natural gamma rays from the formation. Still other systemsmeasure gamma rays in the formation caused by bursts of neutrons intothe formation by a neutron source carried by the tool and pulsed at apreselected interval.

In these nuclear well logging systems, reliance is made upon thephysical phenomenon that the magnitude of energies given off by anucleus resulting from natural radioactive decay or induced nuclearradiation is indicative of the presence of certain elements within theformation. In other words, formation elements will react in predictableways, for example, when high energy neutrons on the order of 14.2 Mevcollide with the elements' nuclei. Different elements in the formationmay thus be identified from characteristic gamma ray energy levelsreleased as a result of this neutron bombardment. Thus, the number ofgamma rays at each energy level will be functionally related to thequantity of each element present in the formation such as the elementcarbon which is present in hydrocarbons. The presence of gamma rays at a2.2 mev energy level may, for example, indicate the presence ofhydrogen, whereas predominance of gamma rays having energy levels of4.43 and 6.13 Mev, for example, may indicate the presence of carbon oroxygen.

Accordingly, in these nuclear well logging systems, it is frequentlyuseful to not only detect occurrence of such particles, but to furtherobtain data regarding their spectral distributions both in terms of timeand energy. Such data can yield extremely valuable information about theformation, such as identification of lithologies which are potentiallyhydrocarbon producing. Moreover, this desired spectral data may not belimited to that of natural gamma rays for example, but also may bedesired for the gamma ray spectra caused by bombardment of the formationwith the aforementioned pulse neutron sources.

Prior art well logging systems for conducting spectral analysis ofnuclear particles have conventionally included a subsurface well logginginstrument which traverses a well borehole. The instrument typicallyincludes a gamma spectrometer including a thallium-activated sodiumiodide crystal optically coupled to a photomultiplier tube. A highvoltage supply accelerates deuterons into a tritium target, generating alarge number of 14.2 Mev neutrons, this pulsed neutron source beingactivated at repetition rates of 20,000 bursts per second. Subsequentgamma radiation from the formation incident upon and detected by thishigh resolution scintillation crystal generates a pulse of light whichin turn causes the photomultiplier tube to generate electrical pulseseach proportional to the gamma ray energy causing the pulse. Thescintillation spectrometer, comprised of the detector-photomultipliertube, is maintained at a low temperature in thermal isolation in aDewar-type flask.

As the photomultiplier tube generates these electrical signals, adownhole electronic amplifier provides voltage amplification andtransmits the detector voltage pulse signals in analog form uphole on asingle or multi-conductor logging cable to surface instrumentation foranalysis and storage. At the surface, this pulsed information isamplified and routed to a multi-channel spectral analyzer system forderiving the desired spectra. The surface multi-channel analyzerprovides a total pulse count and selects pulses within prescribed energywindows for separate counting. In addition, the multi-channel analyzermay also generate a time spectrum of the detected gamma ray pulsesyielding pulse counts within prescribed time windows. In one variationon the aforementioned systems, rather than sending the actual analogvoltage pulses to the surface from the downhole spectrometer, in someinstances systems are provided wherein each pulse is first digitizeddownhole, and the digitized value of the height of each pulse istransmitted to the surface for analysis.

One particular form of well logging system for performing spectralanalysis of induced gamma rays is the carbon-oxygen log wherein a ratioof detected elements is used to indicate presence or absence ofhydrocarbons, among other parameters. In this system, the aforementionedpulsed neutron source produces high energy neutrons which are deliveredinto the formation. As previously noted, when carbon and oxygen arebombarded, as with other elements, by high energy neutrons, they emitgamma rays characteristic of their respective nuclei, with the carbongamma ray energy being 4.43 Mev and the predominant oxygen gamma raybeing 6.13 Mev. In like manner to the other systems, the gamma rays aredetected downhole by a scintillation spectrometer calibrated to countpulses in the energy ranges most indicative of carbon and oxygen.Information available in the spectrum analyzed includes information ofthe inelastic gamma rays and calcium and silicon. Also, after theinelastic reactions have ceased, measurements of the gamma rays ofcapture of silicon and calcium are made.

Neutrons of sufficient energy to excite a carbon or oxygen nucleus arefound to exist in a subsurface formation for only a brief period oftime. Accordingly, the detector is gated and synchronized to make ameasurement while neutrons are being emitted from the source. Acarbon/oxygen ratio is derived by taking a ratio of the gamma ray countsin the selected energy windows.

The gamma ray measurements are presented in a conventional well logformat comprising continuous plotter tracks. One track is used tomonitor the output of the neutron source. Adjacent tracks contain acarbon/oxygen ratio curve, a silicon/calcium ratio curve, and aninelastic calcium/silicon ratio curve.

Well logging systems for measuring neutron absorption in a formation usea pulsed neutron source providing bursts of very fast, high-energyneutrons. Pulsing the neutron source permits the measurement of themacroscopic thermal neutron absoption capture cross-section (Σ) of aformation. The capture cross-section of a reservoir rock is indicativeof the porosity, formation water salinity, and the quantity and type ofhydrocarbons contained in the pore spaces.

Neutrons leaving the pulsed source interact with the surroundingmaterials and are slowed down. In a well logging environment, hydrogenin the surrounding water and hydrocarbons act to slow the neutrons.After the neutrons have been slowed to the thermal state, they arecaptured by atoms in the surrounding matter. Atoms capturing neutronsare in an excited state; and after a short time, gamma rays are emittedas the atom returns to a stable state.

The number of gamma rays present at any time is directly proportional tothe number of thermal neutrons, i.e., the thermal neutron population.The decay rate of this neutron population is an exponential function,and is defined by specifying the time required for the thermal neutronpopulation to decrease to one-half. This time is referred to as aneutron "half-lifetime". While it is actually the neutron lifetime thatis measured, the more useful parameter is the capture cross-section.Capture cross-section and neutron lifetime are inversely related, withcapture cross-section being a measure of the rate at which thermalneutrons are captured in the formation. Analysis of formation in thismanner is referred to as "neutron decay analysis".

The measurement of neutron population decay rate is made cyclically. Theneutron source is pulsed for 20-30 microseconds to create a neutronpopulation. Since neutron population decay is a time-related function,only two time referenced gamma ray count measurements are necessary. Thecapture gamma rays are normally detected from time intervals that are400-600 microseconds and 700-900 microseconds after each neutron burst.As the neutron source is pulsed and the measurements made, thesubsurface well logging instrument is continuously pulled up theborehole.

The recorded log consists of four curves or tracks on a plotter. Thecapture gamma rays measured during the first measurement time period arerecorded on one track. The capture gamma rays measured during the secondmeasurement time period are recorded on a second track. On the third andfourth tracks, there are recorded a monitor of the neutron source outputand the calculated capture cross-section. Capture cross-section iscontinuously calculated from the measurements made during the tWomeasurement time periods.

Along with the thermal neutron log, an epithermal neutron log may besimultaneously recorded. Also, casing collars may be recorded.

Detailed discussion of such a carbon/oxygen digital well logging systemas well as general theoretical background as to such logging operationsmay be found in U.S. Pat. No. 4,471,435 entitled "Computer-Based Systemfor Acquisition of Nuclear Well Log Data" to Meisner, and Carbon/OxygenLog, Publication No. 9417 of the Dresser Atlas Division of DresserIndustries, Inc., and comprising a collection of eight technical paperson the subject, both of which are incorporated herein by reference forall purposes.

The prior art nuclear well logging systems just described, thoughproving to be a very valuable tool in oil and gas exploration, havesuffered from numerous deficiencies. First, with respect to the analogsystems Which transmitted analog voltage pulses from the downholespectrometer to the surface corresponding to each detected gamma ray,serious problems were encountered in pulse distortion and degradationdue to limited band width on the conventional logging cables. Even withthe previously described systems incorporating downhole digitization ofeach spectrometer pulse in an effort to avoid this pulse distortion, thesystem still transmitted the digital values for each pulse uphole,resulting in extremely slow system throughput. Due to the downholeinstrumentation constraints of high temperature environments, low poweravailability, logging tool size constraints, and low signal-to-noiseratios, the approach of deriving downhole spectra was largely thought tobe impractical if not impossible. Nevertheless, a well logging systemand particularly a nuclear well logging system was highly desired whichnot only solved the pulse distortion and throughput problems, butprovided better logging cable utilization which did not require thededication of logging cable conductor time to sending the actualparameter values for each detected gamma ray pulse. It was furtherhighly desirable to provide a nuclear well logging system with improvedresolution, statistical accuracy, calibration and calibrationmaintenance characteristics. Still further, such a system was highlydesirable which could, at the same time, provide for programmed downholesystem flexibility as well as the opportunity for operator adjustment ofparameters such as those affecting spectral generation includingdiscriminator levels, detector gains, gate positions, source tracking,and temperature correction, as well as the potential for downholespectral analysis, enhancement, and data manipulation under control fromthe surface or subsurface. It was still further desirable tosimultaneously generate in addition to any one or a plurality of energyspectra such as the capture and inelastic spectra, various timedistribution spectra for determination of capture cross-section,porosity, or the like. Moreover, it was further desirable to provide asystem wherein the time spectrum of the neutron souce burst itself couldbe acquired. The present invention is directed to achieving these endsand in the promotion of consistent reproducible well logging spectraldata at the surface.

SUMMARY OF THE INVENTION

In accordance with the present invention, a computer-based well loggingsystem and method is provided for acquiring nuclear well logging data,including derivation in a downhole logging instrument of spectralinformation relating to energy and time-distribution either alone or incombination of nuclear particles detected within a subsurface earthformation.

The system includes a subsurface well logging instrument suspendedwithin and adapted to traverse a well borehole and a surface systeminterconnected to the instrument by a suitable communication link suchas a single or multi-conductor logging cable.

The surface system desirably includes a master controller or computerwith associated storage or memory, one or more forms of visual displaysuch as a plotter, and input/output device for communicating with thecontroller, and a conventional modem for communications interfacebetween the surface system and the instrument. The surface system servesthe purpose of acquisition, storage, and display of data generated bythe instrument as well as providing data and command control functionsto the instrument via the communication link.

As the subsurface instrument traverses the well borehole, a depthindicator provides signals indicative of the depth and rate of travel ofthe instrument within the borehole. In response thereto, the controllerproduces periodic depth command signals at prescribed depth intervalssuch as every quarter of a foot which may be used as command signalsconveyed to the instrument for purposes of retrieving spectral datagenerated by the instrument within each such depth interval.

The subsurface instrument includes a detector for detecting natural orinduced gamma ray emissions from subsurface formations which produceselectrical pulses. Each pulse corresponds in time with the incidence ofa corresponding gamma ray on the detector and has an analog voltageamplitude correlative to the energy level of the gamma ray. If thesystem is employed for spectral analysis of neutron-induced gamma rays,the instrument will further include a neutron source for repeatedlyinducing bursts of neutrons into the formation at a preselectedfrequency such as 20,000 KHz.

A multi-parameter analyzer is provided within the instrument foraccumulating these indications of the time of occurrence and energylevels of detected gamma ray pulses occurring during prescribed timeintervals and conveyed from the detector to the analyzer. A memorywithin the analyzer is divided into one or more pluralities of memorylocations, each plurality corresponding to a different spectrum, whethera time-distribution or energy spectrum.

With respect to a time-distribution spectrum, each memory locationuniquely corresponds sequentially to a different time window or channelhaving a preselected discrete time width and is employed to accumulate acount of gamma rays occurring within that particular time window duringa preselected time interval. In one embodiment, these windows will bereferenced to the time of firing of the aforementioned neutron source.

In like manner, for a given desired energy spectrum, each memorylocation uniquely corresponds sequentially to a different adjacentenergy window or channel having a preselected discrete energy width, andis utilized to accumulate a count of gamma rays occurring within thatparticular energy window during a preselected time interval.

The analyzer further includes pulse height analyzer circuitry forgenerating digital representations of the energy levels of the detectedgamma rays and reference clock circuitry for sequentially generatingmemory address codes each corresponding to the sequence of memorylocations for each time spectrum to be acquired. Each time a gamma rayis detected, the thus-generated time window address code present duringthat gamma ray's detetion is employed to address the correlative memorylocation for each time-distribution spectrum corresponding to that timewindow during which the gamma ray occurred. The count in each of thesememory locations is thence incremented by one. Similarly, each time adigital representation of the energy level of a detected gamma ray hasthus been generated, a central processing unit or CPU within theinstrument generates address codes corresponding to the energy windowsor channels in each defined energy spectrum having an energy range whichthe particular digital representation falls within. These address codesare thence employed to increment the count resident in each of theseenergy channels or memory locations.

The multi-parameter analyzer thus automatically accumulates counts forboth the time and energy spectra by the aforementioned address codegeneration. A high speed first-in-first-out buffer interposed betweenthe digital-to-analog converter and memory of the analyzer temporarilystores these counts of pulse height and arrival time data for lateraccumulation in the analyzer memory to the enhance data acquisition rateof the analyzer. A direct memory access bus is provided between theinstrument's CPU and the memory of the analyzer. In this manner, the CPUaccesses the spectral data thus being acquired by the memory of theanalyzer, either under downhole or surface control as desired, such asupon occurrence of depth interrupt commands, without affectingacquisition of the spectral data.

The CPU Will periodically acquire this spectral data from the analyzermemory as desired for transmission to the surface, storage in CPUmemory, or downhole analysis, also as desired.

A feature of the present invention is the flexibility provided by theinstrument CPU, either alone or in response to surface-generatedcommands from the surface CPU, in controlling and defining the variousspectra being generated downhole as well as the related parameters foraccomplishing this function.

With respect to surface control, prior to or during the loggingoperation, signals may be downloaded from the surface to subsurface CPUdefining, for example, the number of spectra desired to be acquired(e.g., time-distribution, energy, or both). This initializinginformation may further include upper, lower, and baseline gamma raythreshold or other discrimination constraint information, the number ofenergy and time window channels and their widths for each desiredspectrum, and the overall time interval for a given borehole depthduring which a given energy or time spectrum is to be acquired.

With respect to the latter feature, the present system contemplatesgeneralized application to various forms of nuclear well logging. Forexample, in performing spectral analysis of naturally-occurring gammarays, the aforementioned energy and/or time-distribution spectral countsmay be permitted to be acquired in the memory of the analyzer for apredetermined time interval corresponding to a distinct increment ofborehole, whereupon these accumulated counts of the energy and/or timespectra may be transmitted to the surface. As the instrument traversesto a different borehole increment, the memory locations corresponding tothe spectra may be cleared in response to the instrument CPU, and yetadditional spectra may be acquired for a subsequent time intervalcorresponding to a different borehole elevation or increment.

In like manner, with respect to spectral analysis of neutron-induced orpulsed neutron sourced gamma rays, with each neutron burst inelasticscattering gamma rays are conventionally produced and detected duringthe first time interval, whereas capture gamma rays are produced anddetected during a subsequent second time interval. In such anapplication, an energy or time-distribution spectrum for both maydesirably be acquired after each source firing and for each timeinterval, e.g., an energy and/or time spectrum may be produced, asdesired, after each source firing for the inelastic gamma ray timeinterval as well as for the capture gamma ray time interval. In thisinstance, the accumulated counts in the memory locations correspondingto the spectra may desirably be allowed to accumulate over a repeatednumber of source firings while the instrument is adjacent one incrementof borehole wherein gamma rays occuring during the first time intervalare utilized only to increment counts in one set of memory locationscorresponding to an inelastic spectrum. In like manner, pulse height orarrival time indications occuring during the second time interval aftersource firing may be utilized only to increment counts in other memorylocations corresponding to a capture gamma ray spectrum, whether anenergy or time-distribution spectrum as desired. These inelastic orcapture spectra, whether energy or time related may then be transmittedto the surface after being acquired over a preselected number of sourcefirings or time interval as desired, which may correspond to generatedspectra functionally related to an increment of borehole, whereuponthese memory locations will be cleared in preparation for acquisition ofsubsequent like-spectra corresponding to a different borehole elevationor increment or a subsequent overall time-interval. Due to theaforementioned address code generation corresponding to time ofoccurrence of detected gamma rays, the subsurface CPU provides a gatingor routing function routing gamma ray energy or time indications to theappropriate inelastic or capture spectrum being acquired. Also, both theenergy level and occurrence time of each detected gamma ray is beingused to generate both time and energy spectral data during the sameborehole pass.

In yet another application, it may be desirable to accumulate a totalenergy or time spectrum acquired after each neutron source firing inwhich case separate memory register in the memory of the analyzer willnot be set up by the CPU to acquire separate spectra each related to thetwo aforementioned inelastic and capture time intervals after eachsource firing. Rather, all such detected gamma rays after a particularfiring of the source will be accumulated in one energy ortime-distribution spectrum and associated memory locations for eachspectrum. In addition to the aforementioned spectrum formationinformation transmitted to the instrument from the surface relating tothe number of channels and the like defining each spectrum, thisinformation may include information defining the time of occurrence andlength of time of each of the first and second time intervalscorresponding to the inelastic and capture gamma rays relative to thesource firing for purposes of the subsurface CPU correspondinglyadjusting the memory locations in the analyzer memory, such inelastic orcapture time interval gating being variable as desired.

Another feature of the present invention is provision for the subsurfaceCPU to analyze the various spectra being generated and to automaticallyadjust the logging operation in response thereto, or to perform asubsurface generation of a function of such acquired spectra.Accordingly, the subsurface CPU may perform analysis of atime-distribution spectrum acquired by the analyzer during the sourcefiring for purposes of source tracking by detecting the source spectrumpeak. An automatic gain control or source firing or inelastic-capturegate adjustment signal may then be generated in response thereto. Thus,the instrument is provided with a gain control circuit whereby inresponse to a digital gain control signal generated by the subsurfaceCPU, the gain control circuit will generate a high voltage supplyfunctionally related thereto which supplies power to the photomultipliertube of the detector. Because the entire time spectrum of each singlesource firing is capable of being acquired the thermal neturon decay maythus be derived between source bursts. Similarly, the spectra thusgenerated subsurface by the instrument may be analyzed and processed bythe subsurface CPU to derive, for example, parameters including themacroscopic thermal neutron absorption capture cross-section of theformation at borehole elevations corresponding to the locations fromwhich such spectra were derived.

BRIEF DESCRIPTION OF THE DRAWINGS

The written description setting forth the best mode presently known forcarrying out the present invention and of the manner of implementing andusing it is provided by the following detailed description of thepreferred embodiment which is illustrated in the attached drawingswherein:

FIG. 1 is an overall schematic diagram of the nuclear well loggingsystem of the present invention;

FIG. 2 is a representative display of neutron decay analysis data, thecapture gamma ray energy spectrum acquired by a well logging system inaccordance with that illustrated in FIG. 1;

FIG. 3 is a typical display of two time spectra which may be generatedby the present system.

FIG. 4 is a representative display of another example of timedistribution analysis data acquired by a well logging system inaccordance with that illustrated in FIG. 1;

FIG. 5 is a simplified block diagram providing a more detailedrepresentation of the well logging instrument circuitry illustrated inFIG. 1 and in particular, that of the multi-parameter analyzer;

FIG. 6 is a timing diagram illustrating timing and wave forms of thesignals at various locations within the circuitry of FIG. 5;

FIG. 7 is a schematic diagram of the reference clock circuitry of FIG.5;

FIG. 8 is a schematic diagram of the demultiplexer circuitry of FIG. 5;

FIG. 9 is a schematic diagram of the pulse height analyzer circuitry ofFIG. 5;

FIG. 10 is a timing diagram illustrating timing and wave forms of thesignals at various locations within the circuitry of FIG. 9;

FIG. 11 is a schematic diagram of the timing and gating circuitry ofFIG. 5;

FIG. 12 is a schematic diagram of the buffer circuitry of FIG. 5;

FIG. 13 is a schematic diagram of the accummulator circuitry of FIG. 5;

FIG. 14 is a schematic diagram of the central processor unit portion ofthe circuitry of FIG. 5;

FIG. 15 is a schematic diagram of the logic decoder portion of thecentral processor circuitry of FIG. 5;

FIG. 16 is a schematic diagram of the memory circuitry of FIG. 5;

FIG. 17 is a schematic diagram of the modem circuitry of FIG. 5.

FIG. 18 is a schematic diagram of the gain control circuitry of FIG. 5;and

FIG. 19 is a flow chart of a program routine executed by the centralprocessing unit for obtaining data in the data analysis mode of systemoperation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings in detail, and particularly to FIG. 1,there is illustrated a nuclear well logging configuration in accordancewith the present invention. A well 10 penetrates the earth's surface andmay or may not be cased depending upon the particular well beinginvestigated. Disposed within well 10 is a subsurface well logginginstrument 12. The system diagram in FIG. 1 is a microprocessor-basednuclear well logging system using multi-parameter analysis of detectedgamma radiations. Well logging instrument 12 includes a detector system14 and a pulsed neutron source 16. In the preferred embodiment, detectorsystem 14 is comprised of a thallium activated sodium-iodide crystalwhich is coupled to a photomultiplier tube. To protect the detectorsystem from the high temperatures encountered in boreholes, the detectorsystem may be mounted in a Dewar-type flask. Also, in the preferredembodiment, source 16 comprises a pulsed neutron source using adeuterium-tritium reaction wherein deuterium ions are accelerated into atritium target, thereby generating neutrons having an energy ofapproximately 14 Mev. A radiation shield 18, preferably of tungsten orbrass, is interposed between source 16 and detector system 14. Cable 20suspends intrument 12 in borehole 10 and contains the requiredconductors for electrically connecting instrument 10 with the surfaceapparatus.

The output from the photomultiplier tube of detector system 14 iscoupled by line 17 into multi-parameter analyzer 22. As will beexplained in greater detail later herein, multi-parameter analyzer 22includes a clock circuit having an output 11 thereof coupled to source16. Thus, source 16 will be pulsed at a repetition rate established bythe clock circuit. Typically, the repetition rate of source 16 will befrom between approximately 20,000 KHz to approximately 50,000 KHz.

Multi-parameter analyzer 22 is also coupled to a central processor unit(CPU) 24 by bus 26. The CPU 24 may suitably be a microcomputer system.CPU 24 is coupled through CPU bus 28 to memory 30, modem 32, and gaincontrol 34. The output of gain control 34 is coupled by line 21 intodetector system 14. Modem 32 is coupled to cable 20 for transmission ofnuclear well logging data over a communication link to the surfaceapparatus.

The surface apparatus includes master controller 33 coupled to cable 20for recovery of the nuclear data from instrument 12 and for transmittingcommand signals to instrument 12. There is also associated with thesurface apparatus depth controller 36 which provides signals to mastercontroller 33 indicating the movement of instrument 12 along theborehole and an indication of the depth of instrument 12 in the borehole10. Teleprinter 38 is coupled to master controller 33 by line 39 toallow the system operator to provide selected input into mastercontroller 33 for the logging operation to be performed by the system.Display unit 40, plotter 42, and mass storage unit 44 are coupled tomaster controller 33 by bus 41. The primary purpose of display 40 andplotter 42 is to provide visual indications of the generated loggingdata as well as systems operation data. Storage 44 is provided forstoring of logging data generated by the system as well as for retrievalof stored data and system operation programs.

In a well logging operation such as is illustrated in FIG. 1, instrument12 is caused to traverse borehole 10 in a conventional manner. It willbe recalled that due to the output 11 of the clock circuit within themulti-parameter analyzer 22, the source 16 will be continuously pulsedat a desired rate which is typically 20,000 KHz. This, in turn, causes aburst of high energy neutrons on the order of 14 Mev to be introducedinto the surrounding formation to be investigated. In a mannerpreviously described, this population of high energy neutrons that areintroduced into the formation will, in turn, cause the generation ofgamma rays within the formation of varying energies and occurring atvarying times which will impinge on the detector 14. As each gamma raythus impinges upon the crystal-photomultiplier tube arrangement indetector system 14, a characteristic voltage pulse having an amplitudefunctionally related to the energy of the particular gamma ray isdelivered on line 17 to the multi-parameter analyzer 22. It will berecalled that both the time distribution spectrum as well as the energydistribution spectrum of these generated gamma rays contain a wealth ofinformation about the characteristics of the formation beinginvestigated, and thus the purpose of shield 18 is to insure that thegamma rays being detected are from the surrounding formation and not dueto neutron bombardment of the detector 14 directly from the source 16.

The purpose of multi-parameter analyzer 22 may be more clearlyunderstood with reference to the illustrative gamma ray spectraindicated in FIGS. 2 through 6. From the foregoing, it will be recalledthat each element in the formation when excited by high energy neutronswill emit gamma ray radiation having energies characteristic of theparticular element. In like manner, the time distribution of theoccurrence of each of these gamma ray pulses can also yield extremelyvaluable information. As but one example, the peak of the timedistribution of detected gamma rays shown in FIG. 4 may be used toperform tracking of the source 16. As but another example, in the systembeing described, a certain amount of "dead time" is experienced due tofinite time required for analog-to-digital conversion of gamma ray pulseheights and the like which adversely affect the spectra being derived.Derivation of a gamma ray time distribution spectrum such as that shownin FIG. 4 which includes time in which the source fires as well as toeither side thereof would enable corrections for this dead time in thespectra. As a final example of the use of a gamma ray time distributionspectrum, such a spectrum permits inference of the neutron decayspectrum or flux in the formation of the gamma rays thus received due tothe source burst. This information, in turn, will permit inferring themacroscopic neutron absorption coefficient or capture coefficient.

From the foregoing, it will thus be noted that it is desirable to havethe capability of generating energy and time distribution spectra forthe gamma rays being detected by the detector 14, and thus themulti-parameter analyzer 22 is provided for this purpose. FIG. 2indicates a typical gamma ray capture energy spectrum generated byanalyzer 22. It may be observed from FIG. 2 that the ordinate representsthe number of gamma rays being detected by the detector 14 at energiescorresponding to discrete channel numbers indicated along the abscissa.More particularly, the presence of hydrogen may thus, for example, beseen as a peak in the gamma ray account occurring in channel 64 whichcorresponds to 2.2 Mev gamma rays.

In like manner, with respect to FIGS. 3 and 4, which are timedistribution spectra for observed gamma rays by the detector 14, as thesource 16 is triggered by line 11 and becomes increasingly active, thenumber of neutrons irradiating into the surrounding formation per unitof time increases until the maximum burst of the source 16 is reached,after which the count rate of such neutrons decreases. It will beexpected that the gamma ray count detected by the detector 14 wouldobserve a similar peak as in fact evidenced by the peak in FIG. 4. Theabscissa will conventionally be divided into a preselected number ofchannels which, in the data depicted in FIG. 4, is arbitrarily selectedas 250, with each increasing channel number corresponding to a discretewindow of time occurring increasingly farther from a time referencepoint.

As aforementioned, the purpose of the multi-parameter analyzer 22 isthus, in part, to form these energy and time distribution spectra forthe detected gamma rays being delivered to the analyzer 22 on line 17from the detector 14. Prior to discussing the analyzer 22 in greaterdetail with reference to FIG. 5, it will be helpful to consider ingeneral the manner in which these spectra are thus created. It will berecalled that as each individual gamma ray is detected by the detector14, a corresponding voltage pulse appears on line 17, the amplitude ofwhich is functionally related to the energy level of the particulargamma ray. ln the analyzer 22, each such analog voltage pulse isconverted by an appropriate analog-to-digital converter in the analyzer22 to a discrete digital value corresponding to the height of the pulseand thus the energy level of the gamma ray which caused it.Additionally, circuitry is provided in the analyzer 22 for not onlydetecting the occurrence of the peak of each of these voltage pulses,but also the time of occurrence relative or referenced to a start timefunctionally related to the time of firing or energizing of the source16. Thus, for each such voltage pulse and corresponding detected gammaray, a digital representation of the time of occurrence of the pulse aswell as its height is accordingly generated in the analyzer 22.

Discussing only the generation of energy spectra within the analyzer 22for the moment, the gamma ray range for which the energy spectrum isdesired is divided into a plurality of discrete adjacent energy windowsand corresponding channel numbers such as those indicated in FIG. 2. Forexample, the range of 1.78-7.64 Mev may be divided into 167 adjacent orconsecutive energy windows or channels, each 0.035 Mev wide. Theaccumulated count in the 62d channel in the range would then accordinglycorrespond to gamma rays occurring at the 3.92 Mev energy level withinthe 0.035 Mev window width of the 62d channel during a preselected timeinterval, such gamma rays at that energy level being indicative ofpresence of carbon.

Memory locations are provided in a suitable memory in analyzer 22, eachof which corresponds to a different channel number and thus energyrange. These locations have the capability of being incremented byoccurrence of a digital number corresponding to the location's channelnumber. Thus, when a gamma ray of a given energy level is detected bythe detector 14, the corresponding voltage pulse height is digitized bythe analyzer 22 and converted into a digital word or channel numbercorresponding to the energy range in which that gamma ray occurred. Thememory location corresponding to that channel number is thus addressedand incremented by one. Over a period of time during which the energyspectrum is being accummulated, gamma rays will be expected to occur atvarying energy levels corresponding to the elements in the formationwhich generated them. Each such gamma ray of differing energy levels andcorresponding channel numbers, will thus, over the period of time ofgeneration of the spectrum, cause the discrete count of such gamma raysoccurring in the corresponding channel number or bin in the memory to beincremented. Over a period of time, as each of the counts in eachdiscrete bin is accummulated, an energy spectrum such as theillustrative spectrum depicted in FIG. 2 will be formed. Byinterrogating each of the memory locations in the analyzer 22 to obtainthe digital count in each channel number corresponding to the count ofgamma rays detected within that channel number or energy range, a visualindication of the spectrum such as that of FIG. 2 may be created.

The time spectrum generating function of the analyzer 22 such as thatillustrated in FIGS. 3 or 4, will now be discussed in general prior to amore detailed description of the analyzer 22 with reference to FIG. 5.It is sometimes desirable to derive a time spectrum which indicates thetime distribution of gamma rays detected by the detector 14 independentof the energy level of these gamma rays, such as for the reasonspreviously given. In other words, useful information is provided byknowing the count of the total gamma rays detected by the detector 14 atpreselected discrete times or time windows relative to a start orreference time Thus, the analyzer 22 is further provided withappropriate time references for generating digital words correspondingnot only to the amplitude of each detected gamma ray pulse but itsoccurrence in time relative to a start time. In like manner to theenergy spectra derivation just described, the time period over which adesired time spectrum is to be generated may be divided into a pluralityof discrete time windows and corresponding channel numbers.

For example, if it is desired to derive a time spectrum of detectedgamma rays over a 50 microsecond interval (corresponding to a 20 KHzrepetition rate for source 16), this 50 microsecond interval may bedivided into 250 consecutive channels 1-250, each of which is 200nanoseconds in width. Also in like manner to the circuitry in theanalyzer 22 just described for generating an energy spectrum,appropriate memory locations may be provided in a suitable memory eachcorresponding to a different one of these time channels. The purpose ofeach such location is to accummulate a running count or total of theoccurrence of the particular digital word assigned to that memorylocation and corresponding uniquely to one of the time windows orchannels. Each time a gamma ray is detected by the detector 14, the timeof occurrence relative to a start time is formed into a digital wordcorresponding to the address of one of the time channels or memorylocations. The count in that memory location is thence incremented byone. Accordingly, as additional gamma rays are detected and theircorresponding arrival times digitized and stored in their appropriatememory locations or channel numbers, a total count will be generated ineach such memory location corresponding to gamma rays occurring withinthat time window interval and within the time allotted for generation ofthe time spectrum. Thus, also in like manner to the energy spectrumdepicted in FIG. 2, by consecutively addressing each of the memorylocations or channels in the memory of the analyzer 22, a visualindication such as that of FIG. 4 may be derived indicating the timedistribution of occurrence of gamma rays.

Still referring to FIG. 1, the general purpose of the analyzer 22 maynow be summarized. In addition, to deriving unique discrete digitalrepresentions of the energy level and time of arrival of each gamma raypulse detected by the detector 14 and referenced to appropriate energyand time references, the analyzer 22 serves two additional basicfunctions. First, for a given time interval over which an energyspectrum is to be derived and a given energy range for the desiredspectrum, the analyzer 22 will accummulate a total of digital counts ineach of a plurality of energy bins, windows, or memory locations. Eachsuch count will correspond to the number of gamma rays detected as ofthe moment of interrogation of the location which were within the energyrange corresponding to the particular location. In like manner, thesecond additional function of the analyzer 22 for the given timeinterval and energy range over which the time spectrum is to be derived,is to accummulate a total of digital counts in memory locations eachcorresponding to the number of gamma rays detected as of theinterrogation occurring within the time window corresponding to thelocations relative to a reference time. Thus, energy and time spectramay both be accumulated and thus derived downhole as desired, alone orin combination thus avoiding the approach known in the prior art whereinthe actual detected gamma ray pulses either in analog or digitized formare transmitted up the cable 20 for spectral analysis at the surface.Moreover, the energy and arrival time of each gamma ray is used informing the time and energy spectra so they may be formed in one passthrough the borehole.

Still referring to FIG. 1, the CPU 24 may at appropriate timesinternally generated or in response to commands on the cable 20,interrogate by means of the bus 26 the analyzer 22 to retrieve spectraldata as desired for transmission to the surface. This data, retrieved onthe bus 26 will be delivered on the bus 28 to the modem 32 for deliveryto the surface on cable 20. In the alternative, however, it iscontemplated that once the spectra are thus derived, this spectral datamay be stored in memory 30 by the CPU 24 for additional processing priorto delivery to the surface.

It will be appreciated that due to the presence of downhole computingand control capability as well as energy and temporal spectralgeneration capability afforded by the CPU 24 and analyzer 22, anextremely powerful and flexible logging system is thus provided which isnot limited to particular configurations. Thus, the details of thespectra being generated by the analyzer 22 may be adaptively changed bythe CPU through parameters delivered on bus 26 to the analyzer 22, inresponse to either software program control resident in the memory 30 oras a function of control information delivered on the cable 20 from thesurface. As but a simple illustration, the number of channels, energy ortime discriminator widths of each channel, signal threshold levels andthe like employed by the analyzer 22 may be varied at will in responseto control from the CPU 24. The flexibility afforded by this featurewill become more apparent hereinafter wherein a more detaileddescription of the analyzer 22 and CPU 24 is provided.

Yet an additional extremely important feature to be hereinafterdescribed in greater detail relates to the ability of the analyzer 22 toderive such spectra automatically wherein by means of a direct memoryaccess to be detailed hereinafter, the CPU 24 may interrogate the memoryof analyzer 22 to retrieve spectral data for processing, analysis ortransmission without interrupting the spectral acquisition processitself. Without such feature, conventionally while the CPU 24 isretrieving spectral data from the memory 30, ability of the analyzer toderive further portions of the spectra would thus be inhibited inasmuchas the memory, while accessed by the CPU 24 is thus not available forstorage of just-derived additional spectral data by the analyzer 22.High speed FIFO buffering in conjunction with the direct memory accessenables this independent spectral generation simultaneously along withretrieval of such spectral information and additional processing thereofas desired, such buffering to be also described later in greater detail.

As but one additional indication of the benefits derived from processorcontrol of the logging operation being described, in response to theinformation being derived by the analyzer 22, the CPU 24 may desirablyprovide gain control information on the bus 28 to appropriate gaincontrol circuitry 34 which, in turn, will, in one embodiment, vary thehigh voltage supply to the photomultiplier tube of the detector 14 bymeans of control signal 21, thereby controlling the gain of the detector14 as desired. This automatic gain control feature will also hereinafterbe described in greater detail.

Each of the schematic blocks pictorially illustrated in the system ofFIG. 1 will now be described in greater detail. First, with respect tothe multi-parameter analyzer 22, a more detailed schematic block diagramthereof will be seen in FIG. 5 and will now be described with referenceto the timing diagram thereof provided in FIG. 6.

First an overall discussion of the operation of the analyzer 22 depictedin FIG. 5 will be given, followed by more detailed discussion thereof.It will be recalled that one of the purposes of the analyzer 22 is tofirst convert the energy level or pulse height of each analog voltagepulse presented thereto on line 17 and corresponding to a detected gammaray into a more convenient digital form. Thus, a pulse height analyzer62 is provided having an output 62A which is an 8-bit digitalrepresentation of the pulse height of each such analog pulse. This pulseheight 62A is delivered to multiplexer 56. Yet an additional purpose ofthe analyzer 22 is to derive a cumulative count of the number of gammarays occurring in each preselected energy window over a preselectedinterval of time for purposes of deriving the previously describedenergy spectra. Accordingly, at appropriate times to be described inmore detail, each such digital representation of pulse height will passon line 62A through the multiplexer 56, through high speed buffer 58,and be accumulated in appropriate memory locations in the accumulator60.

It will further be recalled that yet an additional purpose of theanalyzer 22 is to accumulate a total count of gamma ray pulses occurringin each of a preselected number of discrete time windows so as togenerate the also-described time spectrum. Thus, a series of sequentialmemory address locations will appear in order on line 52A from referenceclock 52 which will be delivered through multiplexer 56 on line 56Athrough buffer 58, and on line 58A to accumulator 60, the manner ofgenerating such address codes to be described shortly. Each time adigital pulse height representation is presented to the multiplexer 56on line 62A, a unique address code also appears on line 52Acorresponding to the time of arrival of the particular gamma ray causingthe generation of the digital representation on line 62A. This addresscode 52A will thus be utilized to address an appropriate location inmemory residing in accumulator 60 so as to increment the count in thatmemory location in response to the gamma ray pulse received. Because thememory locations in accumulator 60 correspond one to one to each uniqueaddress code on line 52A, and thus each further correspond to one of theunique time windows, the cumulative counts in each of the memorylocations in a corresponding portion of the accumulator 60 will thuscollectively comprise the representation of the desired time spectrum ofoccurrence of gamma rays indicated on line 62A. Signals also to bedescribed later will further be presented to the multiplexer 56indicating such things as whether the gamma ray pulse indication whichwas just received was within an inelastic or capture time window, andwhether an 8-bit value for the address code 52A or the value of thepulse height of the gamma ray on line 62A is to be passed through themultiplexer 56 for ultimate storage in the accumulator 60. In otherwords, due to the presence of a number of data and address signalssimultaneously presented to the multiplexer 56, the basic purpose of themultiplexer 56 is to serve to permit access of only a desired suchaddress, data, or command signal to the buffer 58 for use inincrementing only the appropriate memory location in accumulator 60.

Still referring to FIG. 5, the analyzer 22 is provided with a source orreference clock 50 which will desirably run at a repetition rate atwhich the source 16 is to be fired, which is typically at a 20 KHz rate.In FIG. 6, the wave form of this source-firing clock may be seenrepresented as wave form A, causing the neutron source 16 to fire at 50microsecond intervals. A reference clock 52 is also provided, thepurpose of which is to divide down the 50 microsecond period of thesource clock 50 into shorter time windows such as the 200 nanosecondwindows previously described. The further purpose of this referenceclock 52 is to generate a sequence of 8-bit address codes on line 52A,each corresponding as sequentially generated to successive windows orchannel numbers for derivation of a time spectrum. Accordingly, thisreference clock 52 may, for example, run at a 5 MHz rate. It will beseen from the reset clock wave form J of FIG. 6 that a reset pulse isprovided from clock 50 to the reference clock 52 in order to synchronizethe reference clock 52 to the source-firing source reference clock 50.As previously described, these address codes on line 52A derived bycounting the 5 MHz clock pulses out of reference clock 52 will be usedto address memory locations in the accumulator 60. In this way, pulsecounts in corresponding memory locations in the memory of accumulator 60will be incremented each time a gamma ray pulse and a particular addresson line 52A occur correspondingly in time. These address codes will passthrough the multiplexer 56 as address lines 56A through buffer 58 asaddress codes 58A to the accumulator 60 when the multiplexer 56 isenabled to pass them in the manner to be described. It will be notedthat by dividing the 50 microsecond period of clock 50 into 200nanosecond intervals, 250 discrete channels or windows of 200nanoseconds in width are thus created, however, such relative numbersare a matter of choice depending upon desired resolution and the like.

Now that the overall operation of the analyzer 22 has been explained,description of the operation will be given in more detail. It will berecalled that upon energization of the source 16, a population ofinelastic gamma rays will be created and extinguished rapidly within,for example, a ten microsecond interval simultaneous with firing thesource, followed by a population of gamma rays attributable to thecapture phenomenon. It would thus be desirable to provide indications tothe accumulator 60 as to whether the pulse height or arrival time dataon line 62A or 52A, respectively, correspond to gamma rays arrivingwithin the inelastic or capture windows. Thus, a demultiplexer 54 isprovided for indicating to the multiplexer 56 whether the particulargamma ray indication is a capture or inelastic energy value. Under CPU24 control, the reference clock 52 receives on main I/O bus 26 anindication that, for example, the first twenty clock cycles in the clock52 will be considered as a time period in which capture pulses have beenreceived, and that thereafter pulses are considered to be from inelasticscattering. Accordingly, the clock 52 will wait an appropriate timebefore generating the start gate indicated as wave form B in FIG. 6 anddelivered to the demultiplexer 54. The demultiplexer 54 will, in turn,generate two wave forms shown in FIG. 6 as memory route or "MR" Cdelivered to multiplexer 56 and a gating pulse D delivered to AND gate66. The purpose of memory route C is to provide an indication throughthe multiplexer 56, buffer 58, and ultimately to the memory inaccumulator 60 as to whether values presented to the multiplexer 56 areto be stored in the capture or inelastic memory portions of theaccumulator 60. Comparison of wave forms C and D will indicate that whenthe memory route signal C is high and the gate signal D is high for theshortest period of time, the inelastic or total count gate is consideredto be on. Alternatively, when the memory route C is low and the gate Dis high for the longest period of time, the capture gate is consideredto be on.

Referring back to FIG. 6, a data not ready signal E is generated by thepulse height analyzer 62 and delivered to timing and gate control 64.Reference to FIG. 6 indicates that this wave form E is such that it willgo low upon arrival of a gamma ray pulse, and upon conversion of thepulse to digital form in the pulse height analyzer 62 (generally withina nominal 1.5 microsecond time period), the wave form E will go highindicating the conversion has been completed and it is time to store thepulse height. Accordingly, timing and gate control circuitry 64 isprovided for generating pulses to be delivered to the multiplexer 56indicating which input to the multiplexer 56 is to be passed through thebuffer 58 and ultimately stored in accumulator 60, e.g., the purpose ofthe timing and gate control circuitry 64 is to instruct the multiplexer56 whether a pulse height value occuring during the inelastic or capturegate and presented to the multiplexer 56 is to be stored, or whether, inthe alternative, the pulse arrival time of the particular gamma raypresented to the multiplexer 56 is to be stored in the buffer 58 andultimately the accumulator 60. Thus, referring to the timing diagram ofFIG. 6, the control circuitry 64 will generate pulse arrival timestorage pulses at the falling edges of wave form E corresponding toinstructions to store the pulse arrival time for pulses arriving in theinelastic and capture windows, respectively. In like manner, the controlcircuitry 64 will generate pulse height arrival storage pulses indicatedas wave form H on FIG. 6 on the rising edges of the wave form Einstructing the multiplexer 56 to pass pulse height data on line 62Aduring the inelastic and capture windows, respectively, through to thebuffer 58. It will be noted that the pulse height store signal shown bywave form H is gated through AND gate 66 before delivery as signal 66Ato the multiplexer 56. This gate 66 is controlled by the gate D from themultiplexer 54 which, it will be recalled, may in turn be controlled bya signal from the CPU 24 on I/O bus 26. Thus, the purpose of gating 66is to allow the gating off of PHA store pulses when it is not desired tostore certain pulse height values as, for example, when the source 16has been turned off. Still referring to FIG. 6, store pulse line Ibetween multiplexer 56 and buffer 58 will carry a wave form indicated inFIG. 6 which is a combination of wave forms G and H and simplyschematically indicates that the buffer 58 is instructed to store eithera pulse arrival time or pulse height at the indicated times which arepassed through the multiplexer 56. Finally, the wave form F deliveredfrom control circuitry 64 to the multiplexer 56 is provided for purposesof instructing the multiplexer 56 whether a pulse arrival time or pulseheight value is present to be passed through the multiplexer 56.

Referring now to FIG. 7, the reference clock 52 of FIG. 6 will be seendepicted therein schematically in greater detail. It will be recalledthat one function of the reference clock 52 is to provide sequential8-bit address code words each of which corresponds to a different timewindow for addressing correlative memory locations in the accumulator 60wherein gamma ray counts will be accumulated. This function is performedby dividing down the source firing clock frequency 50 and counting theresultant divided wave form. Thus, with reference to FIG. 7, anoscillator 70 is provided which, in the embodiment being described, maybe operating nominally at 5 MHz. The output of oscillator 70 isdelivered to a divider 72 which may be adjusted manually by switch 78 soas to provide to the clock input of clock 74 a frequency having theperiod of the desired time windows for the time spectrum. As previouslydescribed, by means of the source reference line A and reset line J, theclock 74 is operating synchronously with the source firing clock 50, asmay further be seen from the wave forms of FIG. 6. By dividing down andcounting the period of the source reference A, outputs 52A from theclock 74 will be provided and delivered to the multiplexer 56 Whichsequentially comprise 8-bit words each uniquely associated with a nextsuccessive period or window being provided to the clock input of theclock 74. As an example, with reference to FIG. 7, when the reset clockpulse J occurs resetting the clock to the edges of the source referenceA, the clock 74 will begin clocking at a 200 nanosecond ratecorresponding to the reciprocal of the 5 MHz oscillator frequency 70.After the first 200 nanosecond time window clocking pulse, the output52A will, for example, be the 8-bit digital word 00000000. When thesecond 200 nanosecond clock pulse occurs on the clock input of clock 74,the output 52A will then read 00000001. After the third clock input, theoutput 52 will read 00000010. The output 52A will thus continue toincrement each time an additional time window clock pulse of 200nanoseconds enters the clock input of clock 74 until the decoded valueof the 8-bit word on output 52A reaches 250 corresponding to 250 timewindows or channels (50 microsecond period of source clock 50 divided by200 nanosecond period oscillator 70). These sequential 8-bit addresscodes will be delivered through the multiplexer 56 to the buffer 58.When a gamma ray pulse is detected, as for example during the third 200nanosecond window, an address code will be present on the output 52Aunique corresponding to this window, e.g., 00000010. If it is desired toderive a time spectrum, a pulse arrival time store pulse G will bedelivered from the control circuitry 64 to the multiplexer 56 causingthis unique address code to pass on buses 52A and 56A to the buffer 58whereby the memory location in buffer 58 unique corresponding to thisaddress code and time channel will be incremented. This process willcontinue for a generation of the time spectrum whereby duringcoincidence of a gamma ray pulse detection and a unique correspondingaddress code on output 52A, thereafter the unique memory location inbuffer 58 will be incremented.

It will be recalled from the foregoing that after the firing of thesource 16, a period of time passes in which detected gamma rays areprimarily considered to be due to inelastic scattering, after which anext time period occurs wherein detected gamma rays are considered to beas a result of thermal neutron capture. Indication of which time period,also known as a window or gate, is important for purposes of deriving aninelastic or capture energy spectrum or time-distribution spectrum. Thereason for this is that separate memory sections are reserved in thebuffer 58 and accumulator 60 for accumulating these different spectra,e.g., the inelastic energy spectrum, the capture energy spectrum, andtime-distribution spectra whether for total gamma rays, or as desired,gamma rays only in the inelastic or capture window. Thus, it isnecessary to provide indication to the buffer 58 when a gamma ray isdetected as to whether it occurred during the inelastic or capture gateso that the value of the pulse height or time of occurrence of thisgamma ray increments the memory location in the proper portion of thememory corresponding to the spectrum being generated. Moreover, it isdesirable to be able to selectively adjust the relative lengths of thesetwo inelastic and capture gates. Comparison of FIGS. 5 and 7 revealsthat the reference clock 52, and more particularly, the comparator 76 isin communication with the CPU I/O bus 26 whereby the CPU 24 can deliverto the reference clock 52 an indication of the desired time at which thecapture gate is to start relative to the source firing. This is done bymeans of delivering an 8-bit word on the bus 26 to the comparator 76corresponding to this time which may be varied and delivered to the CPU24 through the logging cable 20 or, in the alternative, may be stored inthe memory 30 prior to commencing the logging operation. In any event,stored in the comparator 76 will be an 8-bit digital word correspondingto the desired time of occurrence of the start gate B relative to thesource firing and reference clock J. It will further be noted that thecomparator 76 is provided with 8-bit parallel input from the output 52Aof the clock 74. In this manner, as the output 52A increments upward, atsome point in time, the 8-bit words on the output 52A and the 8-bit worddelivered on bus 26 to the comparator 76 will be equal, at which timethe comparator 76 will generate a start gate output pulse B. Inelasticand capture gate times will be referenced to this start gate B pulseoccurrence time which may thus be adjustable in time relative to thepeak of the source firing by the means just described in varying theword delivered to the comparator 76 on the bus 26. As will be discussedhereinafter further, the CPU 24, by means of DMA bus 26A can interrogatethe various accumulating spectra in accumulator 60 in order to locate intime the peak of the source burst after a source firing pulse isdelivered to source 16 in order to start the inelastic or total countgate an appropriate time from the source peak, which is typically 15 or20 channels (or 5.6 microseconds) as shown by FIG. 4. This insures thegate is on for accumulating a total count spectrum before the sourcefires. The source peak will vary in time from the source firing pulsedue to temperature variations and the like. Accordingly, it is a featureof the invention to track this source peak by generating a time spectrumof the source firing, analyzing it downhole, and adjusting the inelasticand capture gates in response.

The demultiplexer 54 portion of the multi-parameter analyzer 22 will nowbe discussed in greater detail with reference to FIGS. 6 and 8. It willbe recalled that a primary purpose of the demultiplexer 54 is to providea control signal to the multiplexer 56 so as to permit the multiplexer56 to instruct the buffer 58 as to whether the two 8-bit words which maybe appearing on bus 52A and 62A corresponding to pulse arrival time andpulse height have occurred in the inelastic or capture windows.Comparison of wave forms B, C, and D in FIG. 6 relative to the blockdiagram of FIG. 8 indicates that the demultiplexer 54 is first comprisedpreferably of a one shot multi-vibrator 80 which triggers from the startgate pulse B which is delivered to the one shot 80. The start gate pulseB, it will be recalled, occurs a preselected time interval from thefalling edge of the source reference A which fires the source 16,indication of this preselected time being delivered to the demultiplexer54 from the reference clock 52 on line B. It will recalled that thisstart gate time pulse B is variable with respect to and referenced tothe source reference A by means of varying the 8-bit word delivered onbus 26 to the comparator 76 of reference clock 52. The memory routesignal C thus generated by one shot 80 of the demultiplexer 54 will gohigh for a portion of time corresponding to the inelastic gate(typically 15 microseconds for a 50 microsecond source reference dutycycle), and will go low for the balance of the source reference, e.g.,typically 35 microseconds and corresponding to the capture gate. Thismemory route signal C will be delivered to the multiplexer 56 such thatwhen the signal C is high, the multiplexer 56 has thus received anindication that pulse arrival time or pulse height indications deliveredto the multiplexer 56 on buses 52A or 62A correspond to data receivedduring the inelastic window. In like manner, when the signal C deliveredto the multiplexer 56 goes low, this indicates to the multiplexer 56that any pulse arrival address code or pulse height data being receivedby the multiplexer 56 on bus 52A or 62A, respectively, correspond toaddress or data caused by a gamma ray pulse detection during the capturewindow. In addition to varying the start time of the signal C by meansof varying the start gate pulse B (by, in turn, varying input to thecomparator 76 of the reference clock 52 as previously described), therelative time of the capture or inelastic gates, e.g., the duty cycle ofthe signal C, may be varied by varying the period of the one shot 80 asdesired.

Referring to FIG. 8, the output of the one shot 80 is delivered to anadditional one shot 82 for purposes of generating the gated signal Ddelivered to the AND gate 66 of the analyzer 22. It will be recalledthat the purpose of gating the signal D by means of the gate 66 is togate off accumulation of data during the inelastic or capture times asdesired. The QA and QB outputs of one shot 82 trigger on the leading andfalling edges of the input C and, accordingly, when anded through gate84, result in the desired output gate signal D which is routed to theAND gate 66. Thus, when signal D goes high for a short period of time,this time period corresonds to the inelastic gate being on, whereas whenthe signal D goes high for the longer period of time, this correspondsto the capture gate being on. Both of these times may, of course, begated as desired by signal H into the AND gate 66 for delivery as outputsignal 66A to the multiplexer 56 whereby data may be passed through orinhibited from passing through multiplexer 56 in response to the gatedsignal 66A.

The pulse height analyzer 62 portion of the multi-parameter analyzer 22will now be discussed in greater detail with reference to FIG. 9, whichis a schematic block diagram of the pulse height analyzer 62, and FIG.10 which is a timing diagram corresponding to FIG. 9. First, a generaldiscussion will be given of the purpose and operation of the pulseheight analyzer 62 with reference to the timing diagram of FIG. 10,followed by a more detailed discussion of the operation of thecomponents shown in FIG. 9. The primary purpose of pulse height analyzer62 is to receive an analog voltage pulse on line 17 corresponding to agamma ray detected by detector 14, to convert the height of this pulse(which corresponds to the energy level of the detected gamma ray) to acorresponding digital word, and to deliver this digitized value for thepulse height on bus 62A to the multiplexer 56, thence to the buffer 58,and ultimately to the accumulator 60 for storage in the memory locationof the appropriate energy spectrum being accumulated.

The wave form A shown in FIG. 10 is intended to represent a typicalanalog input signal on line 17 to the pulse height analyzer 62 from thedetector 14. Several characteristics may be noted from this wave form.First, as expected, the amplitude of the signal will vary as a functionof the energy level of the gamma rays detected. An upper leveldiscriminator (ULD) and a lower level discriminator (LLD) value may beselected as a function of the dynamic range of the analog-to-digitalconverter and the like which is selected for digitizing the analog gammaray input signal, as shown by the dotted lines in wave form A. Thus,some of the pulses such as the first pulse of the input signal 17 willnot reach this LLD value, whereas others such as the fifth, will exceedthe ULD value. Indications are desirable as to when the input signallevel 17 is outside the analog range suitable for conversion to digitalform. With reference to wave forms B of FIG. 10, such indications may beprovided by a digital ULD output signal and a digital LLD output signal.Comparison of wave forms B with the input signals A indicates that theULD output or "ULD out" will go low only when the input signal 17exceeds the upper ULD value. Similarly, the LLD output or "LLD out"signal will be low anytime the input signal 17 falls below the LLDthreshold level. These two signals, ULD out and LLD out, may be used toinhibit delivery of the input signal 17 to an analog-to-digitalconverter when the input signal exceeds or falls below the ULD or LLDreference levels, respectively. The purpose of this is to avoid, forexample, saturating the A to D converter or avoiding non-linearities inthe A to D output when exceeding the maximum range of the A to Dconverter in the former case, and in the latter case, in order to avoidconverting analog signals not having a level high enough to meet theinput requirements of the A to D converter for an output with anacceptable signal to noise ratio.

An additional aspect of the wave form A of FIG. 10 may be seen, namelyprovision for a baseline threshold value. It is desirable to convert thepeak of each analog pulse comprising the input signal 17, and this maybe conventionally done by differentiating the input signal in that thedifferential of the signal will be zero at its inflection point.However, in the alternative, the differential relative to an adjustedbaseline threshold taken as zero will change in sign indicating zerocrossing points of the zero input relative to the threshold which may beused for interpolating therebetween to detect the peak of the inputsignal 17. Comparison of the peak detect signal in wave forms D of FIG.10 relative to the portions of the input signal exceeding the baselinethreshold of wave forms A of FIG. 10 indicate that such a peak detectsignal will desirably thus be produced which goes low at the occurrenceof each peak of the input signal 17. This peak detect signal indicationof the occurrence of the peak input signal may thence be used as thevalue of the input signal to hold in a sample and hold circuit as thepeak signal for conversion from analog to digital form, in a manner tobe described hereinafter.

Yet an additional feature of a typical input signal 17 shown in waveform A must be provided for, e.g., a phenomenon of pulse pileup. It willbe recalled that the nature of the input signal 17 is statistical, suchthat gamma rays may be detected simultaneously or in close proximity, asindicated by the third and fourth pulses of the input signal 17 on waveform A. In other words, gamma rays may be detected simultaneously oralmost simultaneously and so close together that due to the conversiontime of a typical analog to digital converter, it is not possible toconvert both values and the first must thus be selected. The third andfourth pulses in the input signal of wave form A indicate this to be thecase wherein the third and fourth pulses occur so close together as tooverlap. Thus, it is desirable to provide yet an additional controlsignal occurring only at the peak of the first of these overlappinginput pulses so as to only convert the peak value of the first pulse, asshown by the "start conversation" signal in wave form D of FIG. 10. Itwill be noted that this start conversion pulse will occur on thenegative going edge of the peak detect signals which corresponds tooccurrence of the peak of the input signals. This start conversion pulsemay be utilized for purposes of triggering conversion of the inputsignal at that time because the peak of the input signal is occurring atthat time. However, it will be noted that due to the overlapping of thethird and fourth pulses in the input signal shown in wave form A, astart conversion pulse is not generated for the fourth negative goingedge of the peak detect signal corresponding to the peak of the fourthinput pulse which overlaps the third input pulse. In this manner, theanalog to digital converter will be provided with sufficient time toconvert the third pulse but will not convert the amplitude of the fourthpulse. Yet an additional reason for not converting the amplitude of thefourth overlapped pulse is that a portion of its amplitude isattributable to that of the third pulse which would render a false valuefor the amplitude of the fourth-occurring gamma ray.

Referring further to the wave forms D of FIG. 10, a "data ready" signalwill also desirably be generated in the pulse height analyzer 62 uponcompletion of conversion to digital form of the pulse height values ofthe pulses in the signal input 17. In other words, the start conversionpulse will trigger an analog to digital converter to convert the peakstored values of the selected input pulses in the signal input, suchconversion time typically taking 1.5 microseconds. After conclusion ofthis conversion time dictated by the particular A-to-D converteremployed, the converted value will thus be available out of theconverter in digital form for storage in the accumulator 60, andavailability of this data may therefore be indicated by the data readysignal in wave form D. Also with reference to wave forms D in FIG. 10,anytime the analog value of the input signal 17 falls below the LLDvalue, it is desirable to reset the pulse height analyzer 62 so that itmay begin again tracking the input signal to determine when it isbetween the ULD-LLD range and when a peak has occurred. Thus, a resetsignal is desirably generated by comparing the signal input level to theLLD level such that when the signal input level falls below the LLDlevel, as schematically indicated by comparison between the signal inputand LLD levels of wave form A and the reset pulse of wave form D, thereset pulses are generated.

In summary, with respect to the overall operation of the pulse heightanalyzer 62, it is desirable to provide circuitry for tracking the levelof the input signal 17 and holding it at the occurrence of peak valuesof the pulses in the input signal. Moreover, with respect to overlappingpulses wherein the aforementioned pulse-pileup problem occurs, it isdesirable to hold this analog value at the peak of the first of suchoverlapping pulses. Finally, with respect to values held by a sample andhold circuit, it is desirable to convert these analog values to digitalforms only when the peak held value is held within the desired ULD-LLDrange.

Comparison of wave forms C in FIG. 10 relative to the input signal ofwave form A indicates that wave form A has the aforementioned desirableproperties of an analog signal to be presented to an A-to-D converter.This comparison reveals that the ADC input will track the varying analoglevel of the input signal 17. However, when the peak of the input signal17 is reached, this peak value will be held until the analog value ofthe input signal drops below the LLD threshold level. Moreover,comparison of the ADC input signal of wave form C corresponding to thethird and fourth pulses of the input signal of wave form A indicatesthat the peak value of only the third pulse is held in the ADC input andthat the ADC input does not thereafter increase to the peak of thefourth pulse due to the aforementioned overlapping between the third andfourth pulses. Next, a comparison of the ADC input of wave form C to thestart conversion pulse in wave form D should be made. Peak values forthe pulses in the input signal 17 will be held as indicated by the ADCinput even when the input pulses are below the LLD threshold or abovethe ULD threshold, as in the case of the first and fifth pulses in theinput signal 17. However, due to the input limitations on an A-to-Dconverter, as aforementioned, it is desirable only to convert the heldmaximum value of the pulses occurring within the LLD-ULD range. Thus, itwill be noted that start conversion pulses occur for the held peakvalues of the pulses, but only for those input pulses whose peaks arewithin the LLD-ULD range. In this manner, only those values for the ADCinput signal occurring simultaneously with the start conversion pulsewill be converted to digital form to avoid converting peak held valuesfor input pulses exceeding the LLD-ULD range.

Now that a general explanation of the desired characteristics of a pulseheight analyzer 62 and its overall operation have been given, referencemay now be made to the schematic block diagram of an implementation ofsuch a pulse height analyzer of PHA with reference to FIG. 9. Comparisonof FIGS. 5 and 9 indicate that the analog input signal corresponding todetected gamma rays is delivered on line 17 to a fet switch 90. Thisanalog input will, at appropriate times, be delivered through the fetswitch 90 to the peak detector 92. Detector 92 is basically a voltagefollower which follows the analog input holding appropriate peak valuesthereof in response to the Q2 output of one shot 96 delivered to thedetector 92. The output of detector 92 will accordingly correspond tothe ADC input in wave form C of FIG. 10 which will be delivered to theinput terminals of an appropriate A/D converter 94. When the A/Dconverter 94 receives a start conversion or SC pulse (corresponding tothe start conversion pulses of wave form D in FIG. 10 and the Q1 outputof one shot 96), the A/D converter 94 will convert these analog inputsout of peak detector 92 delivered to the input of A/D converter 94 atoccurrence of each SC pulse. This, in turn, will result in a twelve bitdigital word 62A corresponding to the value of the analog input (theflat portions of the ADC input of wave form C). Reference to FIG. 5 willshow that this digital version of the peak of the analog gamma ray pulsewill be delivered to the multiplexer 56 from the PHA 62 for ultimatestorage as part of the accumulated spectra in the accumulator 60.

The main purposes of the balance of the circuitry depicted in FIG. 9 areas follows. First, the peak detector 92 must be reset to followsubsequent incoming analog signals from the fet switch 90 any time thecurrent input signal falls below the baseline threshold. Secondly, theinput to the peak detector 92 must be inhibited from receiving inputsand thus following any analog signal corresponding to pulses whichoverlap a preceeding pulse such as the fourth pulse overlapping thethird pulse in the input signal in wave form A in FIG. 10. This explainsthe presence of the fet switch 90 so as to isolate the input signal 17from the peak detector 92 upon occurrence of this overlapping pulse sothat the peak detector 92 holds the peak value of the third pulse asshown in the third plateau of the ADC input in wave form C of FIG. 10.Also, this remaining circuitry of PHA 62 in FIG. 9 is necessary togenerate the start conversion or SC pulse so as to instruct the A/Dconverter 94 when to convert the held peak value in the peak detector92. As aforesaid, this instruction to A/D converter 94 must not beprovided when the stored and held value of the peak detector 92 isoutside the LLD-ULD range, explaining absence of the SC pulse in waveform D for the first and fourth plateaus of the ADC signal in wave formC. There is yet an additional function performed by the balance of thecircuitry in FIG. 9 relative to the previously described LLD, ULD, andbaseline thresholds. It will be recalled that a feature of the inventionis to provide for adjustment of such thresholds as desired undercomputer control, whether by downhole CPU 24 or in response to commandsto the downhole system sent through cable 20. The CPU bus 26 willinterconnect to PHA 62, and, more particularly, to a pair of digital toanalog converters 106 and 114. A digital word corresponding to the ULDvalue will be delivered on the CPU bus 26 to D/A converter 106 forconversion to analog form and delivery to a ULD discriminator 108.Similarly, a digital value delivered on bus 26 to the LLD D/A converter114 will be converted into analog form and delivered to LLDdiscriminator 116. The input signal 17 will be routed to a buffer 98,the output of which is delivered to discriminators 108 and 116. Thesediscriminators will by compare the analog input signal 17 to thecorresponding analog value delivered to the discriminators by theirrespective D/A converters. In other words, discriminator 108 willcompare the analog input signal 17 to the analog value for the ULDthreshold delivered by D/A converter 106 to the discriminator 108. Itwill be recalled this value, which may be selected at will be deliveryof a corresponding digital word on CPU bus 26 to converter 106, will beconverted by converter 106 to analog form. In like manner, discriminator116 will compare the analog input signal 17 to the analog value suppliedby D/A converter 114 which corresponds to the LLD threshold. This analogLLD threshold will, in like manner, be created by the conversion inconverter 114 of the corresponding digital LLD threshold word 26delivered to converter 114. Outputs from ULD and LLD discriminators 108and 116, respectively, will accordingly occur when the input signal 17exceeds or falls below the ULD and LLD threshold levels delivered todiscriminators 108 and 116, respectively, by their corresponding D/Aconverters 106 and 114.

It will further be noted that the output of buffer 98 will also bedelivered to a differentiator circuit 350 for purposes of detecting thepeaks of the input signals 17. Thus, spikes will be generated atoccurrence of the input signal peaks having amplitudes corresponding tothe amplitude of the input signal peaks. These peak spikes will bepassed through a baseline discriminator 100 such that outputs from thebaseline threshold discriminator 100 will occur upon occurrence ofsignal input peaks and only when the amplitude of such peaks exceeds theselected baseline threshold. The falling edge of the peak detect signalout of the baseline threshold detector 100 will clock the Q output of aflip flop 112 high. This Q output is delivered to the aforementionedswitch 90 and the one shot 96 turning the switch 90 off. In this manner,further analog input into the peak detector 92 will be prevented.Referring back to FIG. 10, this may be seen with reference to the thirdand fourth pulses in wave form A as compared with wave form C whereinthe peak value of the third pulse is held by the peak detector 92. Theanalog value corresponding to the fourth peak is thus prevented frompassing through the switch 90 to be followed by the peak detector 92, asevidenced by the fact that the third plateau of the ADC input in waveform C remains flat through the time of occurrence of the fourth pulse.The Q output of flip flop 112, in being routed to the A1 input of oneshot 96, also causes this one shot 96 to fire generating an output Q1which is delivered to the A/D converter 94. This Q1 output may berecognized as the start conversion or SC command pulse in wave form Dcausing conversion of the plateau peak being currently held by thedetector 92 into digital form by the A/D converter 94. The A/D converter94 includes an output indicating a time interval from occurrence of thestart of its conversion process to its completion. During thisconversion time, data will thus not be available on the converter output62A for storage. However, after the conversion time necessary to convertthe analog input to the converter 94 into digital form, the digitaloutput will be available on the B1-B12 outputs 62A for storage. Thissignal indicating whether data is ready on the output 62A of theconverter 94 is referred to as the data ready or DR signal which, uponinversion by the NAND gate 102, is available as a "data not ready" or DRsignal E.

Referring back to FIG. 5, this signal E is delivered from the PHA 62 tothe control circuitry 64. In this manner, the control circuitry 64 willonly generate store commands such as F and G for delivery to multiplexer56 after the A/D converter 94 has had the appropriate amount of time toconvert the analog level corresponding to the detected gamma ray,whether it occurs during the inelastic or capture gate. Because gammarays are detected during both the inelastic and capture windows andconverted during these times, data will be ready for the multiplexer 56as well as storage in the accumulator 60 during both of these windows,thus explaining the data ready signal for its inversion being high asshown in FIG. 6 at two distinct times. It will be noted that the oneshot 96 is further provided with an enable input B2. This input B2results from a NAND by gate 118 of the outputs of discriminators 108 and116, and a further NAND of this output of gate 118 with the DR by NANDgate 104. Thus, the one shot 96 will only be triggered, based upon theenable inputs A1 and B2 as described, the analog level on input signal17 being followed by detector 92 is between the LLD and ULD thresholdscorresponding to discriminators 108 and 116 and further only when asignal out of baseline threshold 100 indicates that a peak is present onthe input signal 17 exceeding the baseline threshold.

In summary then, regarding the operation of PHA 62, when a valid analogdata pulse appears on the input signal line 17, the peak detect signalout of baseline threshold 100 indicates that a peak value is present onthe input line 17, exceeding the baseline threshold, thus triggeringflip flop 112. The Q output of flip flop 112, in turn, shuts off switch90 to prevent further analog input into the detector 92. This Q outputis delivered to input A1 of the one shot 96 indicating theaforementioned presence of a peak value above the threshold. Providedthat the peak value is within the ULD and LLD levels, as detected bydiscriminators 108 and 116, and further provided the converter 94 is notpresently converting an analog value as indicated by the DR signal, thisstatus will be indicated to the one shot 96 by the B2 input resulting infiring of the one shot 96 and conversion of the peak stored in thedetector 92 by the converter 94. When the conversion is completed by theconverter 94, the DR signal goes high which is delivered to the presetinput of the flip flop 112. The purpose of this is so that when theconverter 94 is doing a conversion, the preset input into flip flop 112presets the flip flop so that it is disabled so long as a conversion isbeing performed by the converter 94. When the LLD discriminator 116output goes low resulting in a high level out of gate 118 and when thepreset of flip flop 112 or DR is also high indicating a conversion istaking place, the Q2 output of one shot 96 thereby provides a resetpulse to the peak detector 92 to clear the peak value stored in thedetector 92. Also at this time the Q2 goes high resetting the clear lineof the flip flop 110, e.g., resetting the function of the ULDdiscriminator 108. The circuitry is thus reset and ready to receive anext valid pulse.

A more detailed description of an implementation of the timing andgating control functions provided by the control 64 will now be givenwith reference to FIG. 11. Two flip flops 120 and 122 may be arranged incascade whereby the Q output of flip flop 122 is provided to the triggerinput of a one shot multivibrator 124. It will be noted that theaforementioned reference clock may be provided to the clock input of theflip flop 122 so that it toggles at a rate corresponding to the selectedtime windows such as the 200 nanosecond windows previously described.Toggling at this rate will also fire the one shot 124 during eachoccurrence of one of the windows causing the QB output of one shot 124to go low and the QA output to go high, thus generating the previouslydescribed pulse arrival time store pulse G which is delivered to themultiplexer 56. The purpose of this pulse train G is to instruct themultiplexer 56 as to each time a new successive address codecorresponding to a new time window is being presented on line 52A to themultiplexer 56. Each time the QB output of one shot 124 goes low, the Qoutput of flip flop 122 also goes low. This Q output flip flop 122 isanded with the reference clock by AND gate 352, the output of which isrouted to the multiplexer 56 instructing it to deliver a store pulse Ito the buffer 58 whereby the address code currently residing on the bus52A may be passed through the multiplexer 56 and latched into the buffer58. In summary, when it is required to store a pulse arrival time, themultiplexer 56 must be instructed as to the fact that a time to storeoccurrence of a pulse arrival is present, e.g., a new address codecorresponding to a different time window is present on line 52A to belatched by buffer 58 and the pulse height data on line 62A is not to bepassed through the multiplexer 56 to the buffer 58. Moreover, themultiplexer 56 in addition to being provided with an indication that itis time to receive an address code, must further receive a commandinstruction to have the value of the particular address code latched bythe buffer 58, as indicated by the Q output of flip flop 122.

As previously noted, it is at times desirable to provide a gated pulseheight arrival store pulse so that the function of storing pulse heightdata may be selectively enabled or deactivated as desired. This may beaccomplished by means of gating on or off the pulse height store pulseunder control of an enabling pulse from the CPU 24. Accordingly, inputon the CPU bus 26 indicating whether a pulse height store pulse is to beenabled or gated on or off is delivered through AND gate 126, andinverter 128 to AND gate 130. By varying the input on the CPU bus 26 tothe gate 126 the QB and QB outputs corresponding to wave forms H and Fin FIG. 6 may be gated in response to CPU 24 control as desired.

A preferred embodiment of the buffer 58 portion of the multi-parameteranalyzer 22 will now be described in greater detail with reference toFIG. 12. The buffer 58 is preferably of a high speed first in - firstout or FIFO type. It is a significant feature of the presently describedsystem that the memory in accumulator 60 which accumulates the desiredenergy and time spectra may be accessed both by the CPU 24 as well as bythe remaining portions of the multi-parameter analyzer 22. If thismemory was, at a given time, being accessed by the CPU 24 dataacquisition by the multi-parameter analyzer 22 would thus be impededreducing throughput of the system, inasmuch as the data acquisitioncapability of the analyzer 22 would have to be held in abeyance untilthe CPU 24 relinquishes control of the memory. Otherwise, data valuesbeing generated by the analyzer 22 would be lost during CPU 24 controlof the memory inasmuch as this data would have nowhere to be stored.Accordingly, one of the functions of the buffer 54 is to providetemporary storage for the data being derived by the rest of the analyzer22 until such time that this data can be stored in the appropriatememory locations in the memory of accumulator 60 corresponding to thevarious spectra being accumulated.

Referring now to the buffer 58 as depicted in FIG. 12, this function maybe conveniently provided in the form of three buffer chips 140, 142, and144. Inputs Q0-Q7 to buffers 142 and 144 may be recognized as 8-bit timeor energy data received from the multiplexer output data bus 56A, asshown in FIG. 5. In like manner, buffer 142 and 144 outputs Q0-Q7,recognized as 8-bit time or energy data, pass through the buffer 58 andare delivered to tri-state buffer 170 of accumulator 60 (to be discussedin greater detail) as output 58A. Similarly, the inputs to buffer 140may be recognized as signals I (58B), C (corresponding to signals 56Cand 58D), and F (corresponding to signals 56B and 58C). Also in likemanner, the outputs of buffer 140 are also delivered to a tri-statebuffer 168 of accumulator 60 to be described in greater detail. Ingeneral then, a preferred arrangement for the buffer 58 is such that thefirst buffer 140 temporarily stores and passes to the accumulator 60command signals corresponding to signals F, C, and I, indicating whetherthe data present on lines 56A and 58A occur during the inelastic orcapture window, whether they are pulse arrival or pulse height data, andwhether the system is presently in the inelastic or capture windows. Thebuffers 142 and 144 on the other hand, are for purposes of temporarystorage of the actual time or energy data to be accumulated by theaccumulator 60. A DOR signal is also received from each buffer 140, 142,and 144, which is anded together in the accumulator 60 in a manner to bedescribed. Basically, the purpose of this signal is for synchronizationpurposes to insure that data is not retrieved from the buffers 140, 142,and 144, until the information is available simultaneously in all suchbuffers. Finally, a shift data out signal (C5 and C6) from accumulator60 is anded by gate 146 and delivered to the shift out or SO ports ofbuffers 140, 142, and 144. In response to the C5 and C6 signals from theaccumulator 60, generated at appropriate times to be described, thestored data in buffers 140, 142, and 144 will be shifted out on bus 58Aand the output of buffer 142 to respective buffers 170 and 168 in theaccumulator 60.

Each time a store pulse I is presented to the buffer 58, data is storedtherein which cycles through the buffer 58 in a first in - first outmanner. Each chip of the buffer has a data out ready or DOR output whichwill go high when there is presence of data to be cycled out of thebuffer. The DOR signals from each buffer chip will be delivered to ANDgates 150 and 162 of the accumulator 60. Due to propagation of datathrough the buffer chips 140, 142, and 144 of the buffer 58 at differentrates, the purpose of this DOR signal is to insure that data is availabefor output on all three FIFO buffers of buffer 58 before cycling thisdata out. When this has occurred, the output of gate 150 will go high.It will be noted that a peripheral interface adapter output PBO from theCPU 24 will be delivered through gate 152 to the input of a flip flop154. This output of the CPU 24 will indicate whether an accumulation isbeing done by the accumulator 60 or, in the alternative, whether thesystem is waiting for an accumulation. In other words, a high output outof gate 152 indicates that data is available on the output of buffer 58and that, in response to the PBO command from CPU 24, it is desirable todo an accumulation of this data.

A counter 156 is provided in the accumulator 60 having theaforementioned 5 MHz clock wave form input therein which provides a timebase for the instruction cycle for the circuitry of accumulator 60. Thiscounter 156 is enabled by the Q output of flip flop 154 when theaforementioned conditions are met, e.g., when the PBO output of CPU 24indicates an accumulation is desired and when the DOR output of buffer58 indicate that data is available. Upon occurrence of this condition,the Q output of 154 enables the counter 156 so that it begins countingat a rate corresponding to the 5 MHz clock input Q. The output ofcounter 156 is delivered to a 3:8 decoder 158 which has successiveoutputs C0-C7. Due to the decoding function of counter 158, on eachsuccessive clocking of the counter 156, each successive C0-C7 output ofcounter 158 will go high. Each such output C0-C7 is used as a sequencerto cause various components in the accumulator 60 to be activatedsequentially in a manner to be described. For illustrative purposes, itwill be assumed that a gamma ray having an energy level of 2.2 Mev hasjust been detected. It is therefore desirable to increment theaccumulated count by one which is stored in the memory location inmemory 180 corresponding to counts of gamma rays occurring within anenergy window including the 2.2 Mev level. It will further be assumedthat due to appropriate address and control information being providedto the memory 180 by the buffers 168, 170 and 172 in a manner to bedescribed, the 8-bit count for this energy window is present on theoutput of memory 180. The CO command from counter 158 is a load commanddelivered to counters 190 and 192 instructing them to load the output ofmemory 180 (whether a pulse height time or energy value) into thecounters 190 and 192. The memory 180 serves merely as an accumulator tomaintain the current count of gamma rays in each time or energy window.

After such loading in response to the CO command, the C1 command willnext be generated by the counter 158, which is also delivered to counter192 causing the count thus stored in counters 190 and 192 to beincremented by one. Thus, if the count stored in counters 190-192 wasthree corresponding to occurrence up until now of three gamma rays inthe 2.2 Mev range, the count now resident in counters 190-192 inresponse to the C1 command will now be four. Next, the C2 command willbe generated from counter 158 which is delivered to buffer 194 andcauses the buffer to deliver the new updated count value out of theoutputs of counters 190-192, through buffer 194, and back into memory180 in the same memory location that the previous count was stored in.Next, the C3 command is generated by counter 158. This command is acarry output in the event the previous incrementing of counters 190-192results in an overflow as communicated to flip flop 188 by counter 190.If this condition occurs, presence of the C3 command of the clock inputof flip flop 186 and the Q output of 188 on the J input of flip flop 186causes an output of flip flop 186 to be delivered to gate 166. Next,counter 158 generates a C4 command which is a query as to whether or nota carry situation exists. In other words, the C4 command checks for thiscondition via gate 166. If a carry exists, gate 164 resets the counter156 which causes a recycling back to the CO command. This also causes achange in the address lines to the counters 190-192 so that counter 192starts incrementing as opposed to counter 190. Due to the presence oftwo 8-bit counters 190 and 192, it is thus possible to accumulate up to65,000 counts per channel (either an enery or time window) since a16-bit incrementer is thus provided.

Next, the C5 and C6 commands are generated. Referring back to FIG. 12 itwill be recalled that these commands are anded together in gate 146 togenerate a shift out or SO command to the buffers 140, 142, and 144.This shift out command thus occurs after the incrementing step wherein aprevious memory location in memory 180 has been incremented. Thus,during the presence of the SO command to the high speed buffer 58, thebuffer 58 is commanded to shift out the most recently received data onbus 58A to the accumulator 60. During this time the DOR signal is low,however, if additional data is ready after shifting out the prior data,the DOR signal goes high again and the shifting cycles are repeated.Next, the C7 command is generated by counter 158 and delivered throughAND gate 164 to the clear input of counter 156 to reset it. Thus, thecounter 156 may be reset in two ways, either when an increment cyclethrough C0-C7 of counter 158 is completed or, in the alternative, if acarry situation exists with respect to counters 190 and 192.

The data provided on inputs Q0-Q7 carry address information concerningwhich channel in memory 180 to be incremented. As previously mentioned,in one embodiment 256 channels may be available for accumulating each ofthree spectra: the inelastic, capture, and time-distribution spectra. Ithas been found that 2-kilobits of memory in memory 180 are sufficientfor most applications wherein it is desirable to simultaneouslyaccumulate capture, inelastic, and time-distribution spectra, 512 bitsbeing reserved for each spectrum. It will be recalled that with 16-bitdata to provide for up to 65,000 counts per channel, this corresponds to2-bits per channel, or in other words, with 256 channels of 2-bit data,512 bits per spectrum.

In summary then, the sequential operation of the accumulator 60 is asfollows. During cycle CO, a current count is retrieved from memory 180for the particular channel addressed to the memory 180, whether it beone of the 256 channels of inelastic, capture, or time-distributionspectra. This count data is taken from memory 180 and loaded in counters190-192. During command C1, the counters 190 and 192 increment. Duringcommand C2, a write cycle is performed writing the incremented countthrough buffer 194 back into the memory location in memory 180. Atcommand C3, a carry is performed wherein if the prior count in counter190 is 255, a carry flag is set. During command C4, a carry check isperformed by gate 166, and if the carry flag is set, the chip addressincrements by one, addressing chip 192 and the counter 158 is reset toCO, whereupon a new cycle is performed incrementing the counter 192.During command C5-C6, shift out or SO signal is provided to the FIFObuffer 58 causing a shift out of data on the end of the buffer 58, andfinally, during C7, the sequencer or counter 158 is reset.

The buffers 168, 170, and 172 of FIG. 13 will now be discussed ingreater detail. It will be recalled that a feature of the system beingdescribed is that all three spectra, the inelastic, capture, andtime-distribution, are accumulated by accumulator 60 in one memorysimultaneously and automatically. Moreover, this memory 180 may beshared by two devices, namely the components of the multi-parameteranalyzer 22 including the accumulator 60 as well as the CPU 24 itself.These other components have access to the memory by means of buses suchas 58A and 58B, whereas the CPU 24 has access to the memory via a directmemory access or DMA bus 26. Thus, the buffer 172 may be seen to serveas an address bus buffer having an output which corresponds and isrouted to the DMA address bus to the CPU 26. The buffer 182 is an 8-bitbi-directional tri-state bus transceiver tied to the DMA address bus 26.When it is desirable for the CPU 24 to obtain access to data in theaccumulator 60, this bus acts as a memory chip permitting either a readfrom or write to the memory 180 without causing a previously-describedincrementing cycle. The buffer 172 serves as an address buffer so as topermit addressing by means of the DMA bus 26, the desired memorylocations in memory 180.

Turning now to the CPU 24 portion of the multi-parameter analyzer 22, inmore detail a schematic diagram thereof may be seen in FIG. 14. This CPU24 will be discussed in conjunction with a more detailed schematicdiagram of the memory 30 as shown in FIG. 16. As in a conventionalmicroprocessor based system, the memory 30 will be comprised of readonly memory 230 as well as random access memory 232 and 234. The CPU 24is provided with ports B0-B7 which are interconnected to the ROM andRAMs 232 and 234 for purposes of serving as data and lower order addresslines to the memory 30. A latch 204 is provided to the CPU 24 forpurposes of storing these B0-B7 address codes for use at a later time,the stored equivalent of which are referred to as address lines A0-A7which are, in like manner, delivered from the latch 204 to the ROM 230and RAMs 232 and 234. The CPU 24, in addition, includes address portsA8-A10, which are also delivered to the ROM 230 and RAMS 232 and 234. Inthis manner, the CPU 24 may address memory locations in the memory 30for storage or retrieval of data or program steps in a conventionalmanner. Referring to FIG. 14, the CPU 24 will additionally include 14input-output ports PB0-PB7 and PA0-PA7 for purposes of addressing andaccessing selected devices, chips, and the like via the CPU I/O bus 26and, accordingly, these ports will be connected to the bus 26 in aconventional manner such as through a peripheral interface adapter orthe like when required. An oscillator 200 which clocks a flip flop 202serves to provide the machine cycle for the CPU 24. Additional outputsof the CPU 24 and their functions will be discussed hereinafter withreference to other portions of the circuitry depicted herein.

The twelve address lines B0-B7 and A8-A12 of the CPU 24 allow foraccessing of up to 8,000 bits of data. The programmable input/outputports PB0-PB7 and PA0-PA7 employed with a PIA are single control lineswhich may be brought high or low for purposes of controlling variousdevices in the system in a conventional manner. For example, these portsmay be used to control the modem 32 to transmit or receive or may beused to enable reading of various lines throughout the system. It willbe appreciated that when addresses are available as, for example, online 52A to the multiplexer 56 or, for example, when addresses areprovided on outputs B0-B7 and A8-A12 of the CPU 24, it will be necessaryto decode these addresses in order to obtain chip select or enablesignals to translate address codes to the particular devices to beaddressed. This is done in a conventional manner through logic decodingcircuitry such as that indicated in schematic block diagram form in FIG.15. Each device in the system will have its own unique device addresswhich may be scheduled in a memory map so that for every address inputinto the logic decoding circuitry, an appropriate combination of signalswill be output enabling the appropriate devices. Thus, the loggingsystem of the present invention will be provided with such logicdecoding (not shown in FIG. 5).

Referring now to FIG. 17, the modem 32 portion of the system depicted inFIG. 1 will be described in greater detail. The fundamental purpose ofthe modem 32 is to convert 8 or 16-bit serial data received from thesurface on cable 20 into parallel form for use by the instrument 12 and,conversely, to convert such parallel digital information into serialdigital code for transmission to the surface on cable 20. For thesepurposes a parallel to serial shift register 242 and serial to parallelshift register 244 are provided in the conventional manner in additionto an encoder/decoder 254. Input to and output from the shift registers242 and 244, indicated as B0-B7, will be interconnected to the CPU bus26. The cable 20 will in turn be connected to appropriate digitalreceiver and transmitter circuitry 256 and 258 which are well known inthe art and will include appropriate signal conditioning, amplification,and the like. When in the receive mode, serial digital data delivered bythe cable 20 will be delivered to the receiver 256 and thence to theencoder/decoder 254 wherein the information will be decoded. This serialdata will thence be transferred to the serial to parallel shift register242 and thence out on the CPU bus 26 to the CPU 24. When information ispresent on the CPU bus 26 for transmission to the surface, this datawill be delivered on bus 26 to the parallel to serial converter 244, andthence to the encoder/decoder 254 wherein it will be encoded and thencedelivered to the transmitter 258. Transmitter 258 will appropriatelyamplify and condition the encoded serial data into proper form fordelivery on the cable 20 to the surface.

The automatic gain control feature shown schematically as block 34 inthe instrument 12 in FIG. 1 will now be discussed in greater detail withreference to FIG. 18.

The automatic gain control feature of the instrument 12 will now bediscussed in greater detail. Referring to FIG. 1, it will be recalledthat a gain control circuit 34 is provided which is responsive tocontrol signals on the CPU bus 28 and which generates control signals inresponse thereto to adjust the gain of the detector 14. Reasons forproviding such a system are numerous. As one example, the voltage pulsesgenerated by the photomultiplier tube in the detector 14 typically haveextremely high rise times on the order of 50-100 nanoseconds seconds andare of relatively small magnitudes. The analyzer 22 is provided withanalog amplitude and signal shaping circuitry which conditions thevoltage pulses delivered on line 17 from the detector 14. However, it isdifficult to provide an active circuit which can follow pules having theshort rise times and low magnitudes without adding distortion andundesirable components. Accordingly, it was further desirable to providemeans for using DC levels to change gain without introducing theaforementioned distortion of pulses and for improving signal to noiseratios. Yet an additional reason for desirably shifting the gain of thedetector relates to the spectral analysis being performed. Frequently,it is desirable to adjust gain in order to spread the various spectraover a desired range or number of channels, and to shift such gain by aknown amount or energy for each channel.

Thus, referring to FIG. 18 in greater detail, which is a schematic blockdiagram of a gain control circuit of the present invention, the gaincontrol 34 will be provided with a digital to analog converter 270 whichreceives data and control signals on CPU bus 28 from CPU 24. Inparticular, the signals will include a digital representation of adesired discrete gain level at which the detector 14 is to operate.These digital representations may thus be changed at will under controlof the CPU 24 so as to discretely change the gain of the detector 14.

Upon conversion of these digital gain control words into analog form byconverter 270, the analog version thereof is provided to a buffer 272which may comprise an operational amplifier which generates a discreteconstant voltage reference output in response to these analog controllevels input into the buffer 272. The converter 270 is preferably a12-bit converter such that 4,096 discrete voltage steps may be providedto vary the detector gain 14 by a correlative amount. The referencevoltage out of the buffer 272 is thence delivered to a voltage regulator274 which energizes a step up power transformer 276, the secondary ofwhich, as indicated schematically by high voltage supply block 278provides the high voltage supply to the photomultiplier tube within thedetector 14. A feedback 270c between the high voltage supply 278 and theregulator 274 serves to provide a constant high voltage outputindependent of load and temperature variations and the like functionallyrelated to the reference voltage input into the regulator 274. From theforegoing, it will thus be appreciated that an analog high voltageoutput 278 supplied to the photomultiplier tube of detector 14 will beprovided which varies in discrete analog steps in direct correlation tothe variable digital gain control words supplied to the converter 270 inpredetermined steps. Due to the physical operation of suchphotomultiplier tubes, the high voltage supply 278 thereto will vary thegain of the photomultiplier tube and thus the gain provided to thedetected gamma ray analog voltage pulses in the desired manner.

The overall sequential system operation of the logging system depictedin FIG. 1 will now be described in greater detail with reference to FIG.19. The feature of the nuclear well logging system of the presentinvention is that in addition to simultaneously generating a pluralityof downhole spectra, the system including the parameters related to suchspectra are under control of a downhole microprocessor based systemcontained within the instrument 12 as well as, in the alternative, beingunder control of a master controller 33 located at the surface. FIG. 19is intended to schematically represent in block diagram form a flowdiagram of the computer system represented by CPU 24 in memory 30whereby the various operating steps of the system are controlled.

More particularly, prior to commencing a logging operation, a systemcontrol program and initializing data will be downloaded via cable 20from the master controller 33 to the CPU 24 and stored in memory. Inaddition to the logging operation software, at this time parametervalues will be downloaded which are necessary for operation of thesystem and may include, for example, the desired gain at which thedetector 14 will operate, length of gates for the inelastic and capturegamma ray windows, threshold such as the upper level and lower leveldiscriminator values ULD and LLD, baseline threshold values, the type ofspectra to be generated (e.g., inelastic, capture, and/ortime-distribution spectra), and the like. These parameters may, ofcourse, be selectively adjusted during a logging operation as desired,however, this initialization step will typically be performed prior tocommencing a logging operation.

Once this data has been stored in memory 30, the logging operation maycommence as indicated by block 300 in FIG. 19. The required parametersand data will be retrieved on the CPU bus 28 from the memory 30 by theCPU from time to time during the logging operation and particularly asthe operation commences, as indicated by block 302. For example, digitalrepresentations of the desired gain will be delivered on bus 28 to thegain control 34. In like manner, parameters such as the inelastic andcapture gating values, discriminator thresholds, and the like will bedelivered on CPU bus 26 to the multi-parameter analyzer 22. Also, in theinitialization stage of the logging system, prior to generating data allof the various accumulator and scratch memories and buffers must becleared to ready the system for new data being acquired as functionallyrepresented by block 304. Thus, for example, the memory 180 in theaccumulator 60 of analyzer will be cleared in response to appropriateCPU 24 command signals on buffer 26.

Still referring to FIG. 19, the system 12 will then commence operationwith the traversal of the instrument 12 along the borehole. During thistime, as previously described, gamma ray pulses will be detected by thedetector 14, their arrival times and pulse heights determined from theanalyzer and stored in the appropriate memory locations so as to derivethe desired spectra in the analyzer 22. Inasmuch as a preferred versionof the system is depth-based, depth interrupt commands will periodicallybe generated at the surface and transmitted to the system 12 via thecable 20 indicating that the instrument is at preselected depths withinthe borehole. When such a command is received, as indicated by block306, this is a command to the system 12 to obtain data from the receiver256 in modem 32, as indicated in block 308. Prior to doing so, the CPU24, by system operating software in memory 30, will determine if thecommand is valid as shown in block 312, and if not, will generate acommand delivered to the surface indicating that the command was notvalid so as to inhibit transfer of data in the data buffer.

While still awaiting a valid command signal, the system 12 will thenperform a check of data which has been acquired and formed into aspectrum in the analyzer 22. More particularly, as previously described,one usage of the pulse arrival time or time-distribution spectrumacquired by the analyzer 22 is for purposes of source tracking orlocating the peak of the burst of neutrons from the source 16.Accordingly, as indicated by block 310, the CPU 24 will then interrogatevia the DMA bus 26 in response to program control from memory 30, theappropriate memory locations in the accumulator 60 corresponding to thetime-distribution spectrum to locate the channel having the maximumcount, such as channel 100 as shown in FIG. 4 which corresponds to adiscrete reoccurring time relating to the source firing. The CPU 24 willthen compare the time occurrence of these source burst peaks with theselected gating positions such as the inelastic and capture gates whichhave been downloaded previously as just described to insure that thegates are located in the proper and desired positions relative to theactual source burst peaks as actually determined in block 310. This stepmay be seen schematically indicated as block 318. If the gates haveshifted due to variance in the source burst peak for various reasons,the CPU 24 will calculate, again under system operating software controlstored in memory 30, appropriate adjustments to these gates as shown by320, will store these adjusted gate values, and recycle to block 306into an idle loop waiting for additional commands from the surface, asindicated by block 320.

Referring again to block 312, if a command has been received from thesurface and has been invalidated in block 312 by the CPU 24, thiscorresponds to an instruction to transmit the logging data stored in theanalyzer 22 to the surface. Accordingly, as shown by blocks 316 and 322,the CPU 24 will execute such command transmitting the data and alsotransmit a signal to the surface indicating that the transmit databuffer status is good. The system will thereafter cycle to once againcheck the source burst peak to make adjustments as necessary asindicated by block 310.

It is therefore apparent that the present invention is one well adaptedto obtain all of the advantages and features hereinabove set forth,together with other advantages which will become obvious and apparentfrom a description of the apparatus itself. It will be understood thatcertain combinations and subcombinations are of utility and may beemployed without reference to other features and subcombinations.Moreover, the foregoing disclosure and description of the invention isonly illustrative and explanatory thereof, and the invention admits ofvarious changes in the size, shape and material composition of itscomponents, as well as in the details of the illustrated construction,without departing from the scope and spirit thereof.

The embodiments of the invention in which an exclusive property orprivilege is claimed is defined as follows.

I claim:
 1. A method for acquiring spectral well logging data with alogging tool traversing in a subsurface earth formation, comprising thesteps of:deriving energy measurements corresponding to each of aplurality of subsurface gamma rays; defining a plurality of discreteenergy ranges; deriving a count for each of said energy rangescorresponding to a number of gamma rays having said measured energieswithin said each energy range; deriving, coincident with deriving saidenergy measurements, an arrival time of each of said plurality ofsubsurface gamma rays; defining a plurality of discrete time ranges; andderiving a count for each of said time ranges corresponding to a numberof said gamma rays having said arrival times within each time range. 2.The method of claim 1, further including:deriving a time distrbutionspectrum from said counts for each of said time ranges; and deriving atleast one energy distribution spectrum from said counts for each of saidenergy ranges.
 3. The method of claim 2, wherein said at least oneenergy distribution spectrum comprises at least two spectra from thegroup of an inelastic, capture, and total spectral.
 4. The method ofclaim 3, further including:periodically activating a subsurface pulsedneutron source; and wherein at least a portion of said subsurface gammarays comprise gamma rays produced in response to said source activation.5. The method of claim 4, wherein said counts for said energy and saidtime ranges are accumulated over a time interval including a pluralityof said source activations.
 6. The method of claim 5, wherein said stepsof deriving energy measurements and counts for each of said energy andtime ranges are conducted within said logging tool while said tool istraversing said borehole.
 7. The method of claim 5, including:generatinga depth interrupt signal each time said logging tool has traversed apreselected increment of said borehole; transmitting at least a portionof one of said time or energy distribution spectra from said tool tosaid surface in response to said signal; and resetting said counts inresponse to said signal.
 8. The method of claim 7, wherein saidplurality of discrete time ranges comprises:a first plurality of timeranges when said source is activated and a second plurality of timeranges when said source is deactivated; and wherein said plurality ofdiscrete energy ranges comprises: a first plurality of energy rangescorresponding to said gamma rays measured during said first plurality oftime ranges; and a second plurality of energy ranges corresponding tosaid gamma rays measured during said second plurality of time ranges. 9.The method of claim 8, wherein said counts for said first plurality ofsaid energy ranges corresponds to total gamma ray counts.
 10. The methodof claim 9, wherein said counts for said second plurality of energyranges corresponds to capture gamma ray counts.
 11. A method forgenerating spectral well logging data, comprising the stepsof:generating a plurality of source firing pulses; activating asubsurface pulsed neutron source in response to each of said pulses togenerate a corresponding plurality of high energy neutrons for each saidactivation having a peak number of said neutrons; defining for each saidplurality of neutrons a respective time interval during which eachplurality of neutrons was generated; dividing each said time intervalinto a plurality of time windows; and maintaining a count for each saidtime window of gamma rays detected during said windows.
 12. The methodof claim 11, wherein said time windows are identical for each said timeinterval and referenced to said source firing pulses.
 13. The method ofclaim 12, wherein said counts for each said time window are cumulativeover a plurality of said time intervals.
 14. The method of claim 13,wherein said respective time interval for each said plurality ofneutrons is a preselected constant duration.
 15. The method of claim 14,wherein said step of maintaining a count comprises:generating arepeating time sequence of address codes referenced to preselectedtimes, each said code corresponding to a different one of a plurality ofmemory locations; detecting coincidence in time of each of said detectedgamma rays with a corresponding one of said address codes; addressingwith said one of said address codes a corresponding one of saidplurality of memory locations upon said detecting coincidence of saidgamma rays; and incrementing a stored count in said corresponding one ofsaid memory locations.
 16. The method of claim 15, wherein saidpreselected times correspond to said plurality of said source firingpulses.
 17. The method of claim 16, wherein said stored count isaccumulated for said plurality of source firing pulses.
 18. The methodof claim 17, further including:generating a depth interrupt signal whensaid neutron source is at preselected depths within a borehole; andtransmitting the cumulated counts in each of said memory locations tothe surface in response to said signals.
 19. The method of claim 18,further including clearing said memory locations in response to saidsignals.
 20. The method of claim 19, wherein said time intervalcomprises an inelastic gamma ray gate.
 21. Apparatus for acquiringspectral well logging data from a subsurface earth formationcomprising:detector means for generating electrical indications ofsubsurface gamma ray particles; pulse height analyzer means for derivinga digital representation of an energy magnitude of each of said gammaray particles from said electrical indications; pulse arrival time meansfor deriving, coincident with deriving said energy magnitude, a digitalrepresentation corresponding to an arrival time of each of said gammaray particles from said electrical indications; and storage means forcorrelatively storing said digital representations of said energymagnitude and said arrival time for each of said gamma ray particles.22. The apparatus of claim 21, further including:reference clock meansfor generating a plurality of source firing pulses; and source means forintroducing high energy neutrons into said formation in rsponse to saidpulses.
 23. The apparatus of claim 22, further including:gate generatingmeans for generating a first gate during substantial presence of saidneutrons and a second gate during substantial absence of said neutrons.24. The apparatus of claim 23, further including:address generatingmeans for generating a first numerical sequence of address codes inresponse to each said firing pulse, each said code corresponding to adifferent time interval after one of said pulses.
 25. The apparatus ofclaim 24, further includingcoincidence detector means for detectingcoincidence of said address codes with corresponding ones of saidgenerated indications of said gamma ray particles.
 26. The apparatus ofclaim 25, wherein said storage means includes a first plurality ofmemory locations each selectively addressible by a different one of saidaddress codes.
 27. The apparatus of claim 26, wherein said storage meansincludes a second and third plurality of memory locations eachcorresponding to said digital representations of said energy magnitudesof particles detected during said first and second gates, respectively.28. The apparatus of claim 27, whereineach location of said second andthird plurality of memory locations corresponds to a different energyrange and has a corresponding associated address code; and wherein saidapparatus further includes comparator means for selecting one of saidassociated address codes for each of said digital representations ofenergy magnitude as a function of said energy magnitude and whether saideach digital representation corresponds to a gamma ray detected duringpresence of said first or said second gates.
 29. The apparatus of claim28, further including:depth interrupt means for generating interruptsignals indicative of traversal of said apparatus through preselectedincrements of said borehole; and communication means for retrievingvalues stored in said first, second, and third memory locations of saidstorage in response to said interrupt signals.
 30. The apparatus ofclaim 29, wherein said communication means includes means for clearingsaid first, second, and third memory locations of said storage inresponse to said interrupt signals.
 31. Apparatus for subsurfaceacquisition of well logging pulsed neutron source time spectrum datacomprising:source pulse means for generating a plurality of sourcetrigger pulses; neutron source means for generating a burst of highenergy neutrons after and in response to each of said trigger pulses;address code generator means for generating a numerical sequence ofaddress codes in response to each of said trigger pulses, each said codecorresponding to a different time interval between consecutive ones ofsaid pulses and including ones of said time intervals before and aftersaid burst; detector means for generating electrical indications of thearrivals of subsurface gamma rays in response to said neutrons; storagemeans having a plurality of memory locations each of said memorylocations addressable by a differnet one of said address codes; andgating means for gating of said address codes being generated by saidaddress code generator means to said storage means coincident with saidgeneration of said indications of said gamma rays.
 32. The apparatus ofclaim 31, including incrementing means for incrementing counts in saidmemory locations upon said storage means being addressed by acorresponding one of said address codes.
 33. The apparatus of claim 32,wherein said time intervals are equal in duration and includessubstantially an entire time period between said consecutive ones ofsaid trigger pulses.
 34. The apparatus of claim 33, including means forclearing said counts in said storage means after said plurality ofpulses.
 35. The apparatus of claim 34, wherein said clearing means isactivated in response to a subsurface depth position of said apparatus.